Use of phosphoketolase for producing useful metabolites

ABSTRACT

The present invention provides a bacterium which has an ability to produce a useful metabolite derived from acetyl-coenzyme A, such as L-glutamic acid, L-glutamine, L-proline, L-arginine, L-leucine, L-cysteine, succinate, and polyhydroxybutyrate, wherein said bacterium is modified so that activities of D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase are enhanced. The present invention also provides a method for producing the useful metabolite using the bacterium.

This application claims priority under 35 U.S.C. §119(e) to U.S. provisional application 60/644,562, filed on Jan. 19, 2005, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing useful metabolites, particularly those derived from acetyl-coenzyme A (acetyl-CoA). The present invention also relates to novel bacteria useful in the production method.

2. Brief Description of the Related Art

Conventionally, useful metabolites such as L-amino acids, their intermediates, and other chemicals of bacterial metabolism are produced by methods in which bacterial strains isolated from natural sources, or mutants thereof, have been modified to enhance their productivity.

Sugar is the main source of carbon in a microorganism which is suitable for fermentation. The Embden-Meyerhof and pentose phosphate (pentose-P) pathways are the two preliminary routes of intermediary sugar metabolism in a microorganism. A third route, the Entner-Doudoroff pathway, is also known, as are some of the connections with carboxylic acid pathways. During glycolysis, glucose is metabolized to main intermediate compounds, such as phosphoenolpyruvate, pyruvate, and acetyl-coenzyme A, which are used as constituents in the formation of many cellular compounds, such as L-amino acids, purines and pyrimidines, vitamins etc. Also, generation of energy (ATP and NADH) occurs during glycolysis. Pyruvate formed after glycolysis is often converted back to phosphoenolpyruvate (PEP) by phosphoenolpyruvate synthase encoded by the pps gene, or to acetyl-CoA by pyruvate dehydrogenase encoded by the pdh gene etc. One of the above-mentioned compounds, acetyl-CoA, is formed from pyruvate via pyruvate dehydrogenase, and accompanied by the release of CO₂. This loss of one carbon atom results in decreased production yields of useful compounds derived from acetyl-CoA. Two enzymes of the bifidum pathway, D-xylulose-5-phosphate phosphoketolase (also known as “phosphoketolase”) and fructose-6-phosphate phosphoketolase, have been reported. D-xylulose-5-phosphate phosphoketolase (EC 4.1.2.9) catalyzes the phosphate-consuming conversion of xylulose-5-phosphate to glyceraldehyde-3-phosphate and acetylphosphate, with the concommitant release of one molecule of water. Fructose-6-phosphate phosphoketolase (EC 4.1.2.22) catalyzes the phosphate-consuming conversion of fructose-6-phosphate to erythrose-4-phosphate and acetylphosphate, with the concommitant release of one molecule of water. Both enzymes form acetylphosphate, the precursor of acetyl-CoA, without losing carbon via CO₂. D-xylulose-5-phosphate phosphoketolase (EC 4.1.2.9) has been reported in bacteria belonging to the genera Acetobacter (Schramm, M. et al, J. Biol. Chem., 233(6), 1283-8 (1958)), Bifidobacterium (Sgorbati, B. et al, Antonie Van Leeuwenhoek. 42(1-2), 49-57 (1976); Grill, J. P. et al Curr Microbiol., 31(1), 49-54 (1995)), Lactobacillus (Posthuma, C. C. et al, Appl. Environ. Microbiol., 68(2), 831-7 (2002)), Thiobacillus (Greenley, D. E. and Smith, D. W., Arch. Microbiol., 122, 257-261 (1979)), in yeasts belonging to the genera Candidd, Rhodotorula, Rhodosporidium, Pichia, Yarrowia, Hansenula, Hansenula, Kluyveromyces, Saccharomyces, Trichosporon, Wingea (Evans, C. T. and Ratledge, C., Arch. Microbiol., 139, 48-52 (1984); Ratledge, C. and Holdsworth, J. E., Appl. Microbiol. Biotechnol., 22, 217-221 (1985)). Fructose-6-phosphate phosphoketolase (EC 4.1.2.22) has been reported in bacteria, such as Acetobacter xylinum (Schramm, M. et al, J. Biol. Chem., 233(6), 1283-8 (1958)), Bifidobacterium globosum and Bifidobacterium dentium (Sgorbath, B.et al, Antonie Leeuwenhoek, 42, 49-57 (1976)), Bifidobacterium bifidum, Gardnerella vaginalis (Gavini, F. et al, Anaerobe, 2, 191-193 (1996)), and yeasts, such as Rhodotorula graminis, Rhodotorula glutinis, Candida sp., Candida tropicalis, Saccharomyces pastorianus (Whitworth, D. A. and Ratledge, C., J. Gen. Microbiol., 102, 397-401 (1977)). It has been reported that in some organisms both activities are represented by one enzyme (see, for example, the articles of Schramm, M. et al (J. Biol. Chem., 233(6), 1283-8 (1958)); Sgorbati, B. et al (Antonie Van Leeuwenhoek. 42(1-2), 49-57 (1976)); Meile, L. et al (J. Bacteriol., 183(9), 2929-36 (2001))).

Phosphoketolase genes from two species have been cloned and their sequences determined. These are the xfp gene which encodes D-xylulose-5-phosphate phosphoketolase/fructose-6-phosphate phosphoketolase from Bifidobacterium lactis (Meile, L. et al, J. Bacteriol., 183(9), 2929-36 (2001)), and the xpkA gene which encodes D-xylulose-5-phosphate phosphoketolase from Lactobacillus pentosus (Posthuma, C. C. et al, Appl. Environ. Microbiol., 68(2), 831-7 (2002)). A seach of the Microbial Genome database provided by the National Center for Biotechnology information (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?CMD=&DB=genome) revealed several genes encoding for putative phosphoketolases.

Methods for improving the ability of yeast to produce ethanol from xylose by introducing genes for xylose reductase, xylitol dehydrogenase, and additionally phosphoketolase are known (WO2003078643). However, effects of using the phosphoketolase gene for the elimination of carbon dioxide have never been reported.

SUMMARY OF THE INVENTION

An object of the present invention is to enhance production of useful metabolites by strains of bacteria which have the ability to produce the metabolites as well as provide a method for producing the metabolites using these strains.

It is an object of the present invention to provide a bacterium having an ability to produce an useful metabolite, wherein the bacterium is modified to have an increased activity of D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase.

It is the further object of the present invention to provide the bacterium described above, wherein the useful metabolite is derived from acetyl-coenzyme A.

It is the further object of the present invention to provide the bacterium described above, wherein the useful metabolite is selected from the group consisting of L-glutamic acid, L-glutamine, L-proline, L-arginine, L-leucine, L-cysteine, succinate, and polyhydroxybutyrate.

It is an object of the present invention to provide a bacterium having an ability to produce a useful metabolite, wherein the bacterim inherently does not have an activity of D-xylulose-5-phosphate phosphoketolase or fructose-6-phosphate phosphoketolase, and wherein said bacterium has been transformed with a DNA fragment coding for D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase.

It is the further object of the present invention to provide the bacterium described above, wherein the useful metabolite is derived from acetyl-coenzyme A.

It is the further object of the present invention to provide the bacterium described above, wherein the useful metabolite is selected from the group consisting of L-glutamic acid, L-glutamine, L-proline, L-arginine, L-leucine, L-cysteine, succinate, and polyhydroxybutyrate.

It is the further object of the present invention to provide the bacterium described above, wherein the bacterium is selected from the group consisting of Enterobacteriaceae family, Coryneform bacterium, and Bacillus bacterium.

It is a further object of the present invention to provide the bacterium described above, wherein the bacterium belongs to the genus Escherichia or Pantoea.

It is the further object of the present invention to provide a method for producing a useful metabolite, comprising cultivating the bacterium in a culture medium, and collecting the useful metabolitefrom the culture medium.

It is the further object of the present invention to provide the method described above, wherein the useful metabolite is derived from acetyl-coenzyme A.

It is the further object of the present invention to provide the method described above, wherein the useful metabolite is selected from group consisting of of L-glutamic acid, L-glutamine L-proline, L-arginine, L-leucine, L-cysteine, succinate, and polyhydroxybutyrate.

These objects have been achieved by enhancing an activity of D-xylulose-5-phosphate phosphoketolase or fructose-6-phosphate phosphoketolase in a bacterium which has the ability to produce a useful metabolite, particularly one derived from acetyl-CoA, which has made it possible to utilize carbon more efficiently and increase production of the useful metabolite by the bacterium. Thus, the present invention has been completed.

The present invention is described in detail below.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the alignment of D-xylulose-5-phosphate phosphoketolases from L. pentosus and L. plantarum (identity 98.5%).

FIG. 2 shows the construction procedure of plasmid pBS3.

FIG. 3 shows the construction procedure of plasmid pBS4S.

FIG. 4 shows the construction procedure of plasmid pBS5T.

FIG. 5 shows the growth curves of phosphoketolase gene amplified strains and a control strain on a minimum medium containing acetate.

FIG. 6 shows the growth curves of phosphoketolase gene-amplified strains and a control strain in a minimum medium.

FIG. 7 shows the electrophoresis of cell extracts containing phosphoketolase protein.

FIG. 8 shows the production of L-glutamic acid using phosphoketolase gene-amplified strains under biotin-sufficient conditions.

FIG. 9 shows the production of L-glutamic acid using phosphoketolase gene-amplified strains with the addition of penicillin.

FIG. 10 shows the production of L-glutamic acid using a strain in which a phosphoketolase gene is introduced and 6-phosphofructokinase activity is reduced.

FIG. 11 shows the production of L-glutamic acid using a strain in which a phosphoketolase gene is introduced and 6-phosphofructokinase activity is reduced in the presence of penicillin.

FIG. 12 shows the production of L-glutamine (Gln) using a phosphoketolase gene-amplified strain.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The bacterium of present invention encompasses a bacterium which has an ability to produce a useful metabolite, wherein the bacterium has been modified to increase the activity of D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase.

The bacterium of present invention includes both a bacterium which does not inherently have an activity of D-xylulose-5-phosphate phosphoketolase and fructose-6-phosphate phosphoketolase and a bacterium which inherently has an activity of D-xylulose-5-phosphate phosphoketolase and fructose-6-phosphate phosphoketolase. By introducing a gene encoding D-xylulose-5-phosphate phosphoketolase or fructose-6-phosphate phosphoketolase into such a bacterium, productivity of acetyl-CoA is enhanced in the bacterium, leading to enhanced production of useful metabolites.

In the bacterium of the present invention, either or both of the activities of D-xylulose-5-phosphate phosphoketolase or fructose-6-phosphate phosphoketolase may be enhanced.

In the present invention, the term “phosphoketolase” includes both D-xylulose-5-phosphate phosphoketolase and fructose-6-phosphate phosphoketolase.

In the present invention, the phrase “activity of D-xylulose-5-phosphate phosphoketolase” means an activity which catalyzes the phosphate-consuming conversion reaction of xylulose-5-phosphate to glyceraldehyde-3-phosphate and acetylphosphate with the concommitant release of one molecule of water. This activity can be measured by the method described by Goldberg, M. et al (Methods Enzymol., 9, 515-520 (1966) or L.Meile (J. Bacteriol. (2001) 183; 2929-2936).

In the present invention, the phrase “activity of fructose-6-phosphate phosphoketolase” means an activity which catalyzes the phosphate-consuming conversion reaction of fructose-6-phosphate to erythrose-4-phosphate and acetylphosphate with the concommitant release of one molecule of water. Fructose-6-phosphate phosphoketolase activity can be measured by the method described by Racker, E. (Methods Enzymol., 5, 276-280 (1962) or L. Meile (J. Bacteriol. (2001) 183; 2929-2936).

The activity of phosphoketolase in bacteria which inherently have phosphoketolase activity can be enhanced more than that of a wild-type or non-modified strain, preferably not less than 1.5-fold, more preferably not less than 2-fold, and most preferably not less than 3-fold of a wild-type or non-modified strain. Furthermore, in this invention, a bacterium that inherently does not have activities of D-xylulose-5-phosphate phosphoketolase and fructose-6-phosphate phosphoketolase can be used as a parent strain. The activity of phosphoketolase in the bacterium which inherently does not have phosphoketolase activity can be enhanced by transforming the bacterium with a DNA fragment coding for D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase. Whether a parent bacterium inherently possesses phosphoketolase activity or not can be measured by the same method as described above.

Whether the phosphoketolase activity has been introduced may be confirmed using a pyruvate dehydrogenase (PDH)-defective strain. PDH-defective strains are auxotrophic for acetate. On the other hand, if phosphoketolase is introduced, a PDH-defective strain will not be auxotrophic for acetate any more. Accordingly, introduction of the gene encoding phosphoketolase can be confirmed based on compensation by the host strain for acetate auxotrophy. As a PDH-defective strain, the ΔaceE strain as described in the Examples can be used.

“D-xylulose-5-phosphate phosphoketolase” may be an enzyme derived from bacteria having an activity of D-xylulose-5-phosphate phosphoketolase, including lactic acid bacterium, methanol-assimilating bacterium, methane-assimilating bacterium, Streptococcus bacterium, and especially those bacteria belonging to the genera Acetobacter, Bifidobacterium, Lactobacillus, Thiobacillus, Streptococcus, Methylococus, Butyrivibrio, Fibrobacter, and/or yeast belonging to the genera Candida, Rhodotorula, Rhodosporidium, Pichia, Yarrowia Hansenula, Kluyveromyces Saccharomyces, Trichosporon, Wingea, or the like.

“Fructose-6-phosphate phosphoketolase” may be an enzyme derived from bacteria having an activity of fructose-6-phosphate phosphoketolase that belong to the genera Acetobacter, Bifidobacterium, Chlorobium, Brucella, Methylococus, Gardnerella, and/or yeast belonging to Rhodotorula, Candida, Saccharomyces, or the like.

It is also possible that both activities are represented by one enzyme, D-xylulose-5-phosphate/fructose-6-phosphate phosphoketolase.

The nucleotide sequence of the xpkA gene encoding D-xylulose-5-phosphate phosphoketolase of Lactobacillus pentosus MD363 has been registered in the EMBL/GenBank database under accession number AJ309011 (Posthuma, C. C. et al, Appl. Environ. Microbiol., 68(2), 831-7 (2002)) and is shown as SEQ ID NO: 1. The nucleotide sequence of the xpk1 gene encoding D-xylulose-5-phosphate phosphoketolase of Lactobacillus plantarum has been registered in the EMBL/GenBank database under accession number NC_(—)004567 Region: complement (2362936..2365302) (Kleerebezem, M., et al, Proc. Natl. Acad. Sci. U.S.A. 100 (4), 1990-1995 (2003)) and is shown as SEQ ID NO: 3. The nucleotide sequences of the xpk and xpk homologous genes are shown in Table 1 and the Sequence Listing. The alignment of the amino acid sequences of SEQ ID NOS: 2 and 4 is shown in FIG. 1. TABLE 1 SEQ ID: SEQ ID: GenBank Gene microorganism description EC No. DNA Amino acid Accession No xpkA Lactobacillus pentosus phosphoketolase 4.1.2.9 1 2 AJ309011 xpk1 Lactobacillus plantarum phosphoketolase 4.1.2.9 3 4 NC_004567 complement (2362936 . . . 2365302) xpk2 Lactobacillus plantarum phosphoketolase 4.1.2.9 62 63 NC_004567 Complement (3169067-3171478) xpk Streptococcus agalactiae hypothetical protein 4.1.2.9 64 65 NP_736274. NEM316 gbs1840 ptk Lactococcus lactis phosphoketolase 4.1.2.9 66 67 NP_267658 subsp. lactis Il1403 xpk Lactobacillus johnsonii probable xylulose-5- 4.1.2.9 68 69 NC_005362 NCC 533 phosphate/fructose-6- phosphate phosphoketolase xpk Lactobacillus acidophilus NCFM xylulose-5- 4.1.2.9 70. 71 YP_193510 phosphate-fructose phosphoketolase

The nucleotide sequence of the xfp gene encoding D-xylulose-5-phosphate/fructose-6-phosphate phosphoketolase of Bifidobacterium lactis has been registered in the EMBL/GenBank database under accession number AJ293946 (Meile, L. et al, J. Bacteriol., 183(9), 2929-36 (2001)) and is shown as SEQ ID NO: 5, and the amino acid sequence encoded by the xfp gene is shown as SEQ ID NO: 6. The nucleotide sequences of the xfp and xfp homologous genes are shown in Table 2 and the Sequence Listing. TABLE 2 SEQ ID: SEQ ID: GenBank Gene microorganism description EC No. DNA Amino acid Accession No xfp Bifidobacterium longum xylulose-5- EC: 4.1.2.- 72 73 NP_696135 phosphate/fructose- 6-phosphate phosphoketolase xfp Chlorobium tepidum putative EC: 4.1.2- 74 75 NP_662409 phosphoketolase xfp Brucella suis xylulose-5- EC: 4.1.2- 76 77 NP_699578 phosphate/fructose- 6-phosphate phosphoketolase xfp Brucella abortus xylulose-5- EC: 4.1.2- 78 79 YP_223570 phosphate/fructose- 6-phosphate phosphoketolase

XpkA, xpk1, and xfp genes can all be obtained by PCR (polymerase chain reaction; refer to White, T. J. et al., Trends Genet., 5, 185 (1989)) using primers designed based on the nucleotide sequence of the known genes. The genes containing a motif of phosphoketolase can be obtained using pfam and motif (http://pfam.wustl.edu/, http://www.genome.jp/dbget-bin/www_bget?pfam+XFP). The xpk1 gene from Lactobacillus can be obtained by using the primers shown in SEQ ID No.7 and No.8. The xfp gene from Bifidobacterium can be obtained by using the primers shown in SEQ ID No.11 and No.12. The xpkA gene from Lactobacillus can be obtained by using the primers shown in SEQ ID No.19 and No.24.

The activity of phosphoketolase can be enhanced by transforming a parent strain with a DNA encoding phosphoketolase. Transformation of a bacterium with a DNA encoding phosphoketolase can be performed by conventional methods using a DNA encoding phosphoketolase as described above.

A DNA encoding D-xylulose-5-phosphate phosphoketolase may be a DNA which encodes a variant of D-xylulose-5-phosphate phosphoketolase, which can be found in different strains of bacteria, according to natural diversity. The DNA encoding such variants can be obtained by isolating a DNA which hybridizes with xpkA gene (for example, SEQ ID NO: 1 or 3), or part of the gene, under the stringent conditions, and which codes for a protein having the activity of D-xylulose-5-phosphate phosphoketolase. The term “stringent conditions” as used herein include conditions under which a so-called specific hybrid is formed, and a non-specific hybrid is not formed. For example, stringent conditions may include conditions under which DNAs having high homology, for instance, DNAs having homology no less than 70% to each other, preferably 80% or more, more preferably 90% or more, most preferably 95% or more, are hybridized. Alternatively, the stringent conditions are exemplified by conditions which comprise ordinary conditions of washing in Southern hybridization, e.g., 60° C., 1×SSC, 0.1% SDS, preferably, 60° C., 0.1×SSC, 0.1% SDS. Duration of washing depends on the type of membrane used for blotting and, as a rule, is recommended by the manufacturer. For example, recommended duration of washing of the Hybond™ N+nylon membrane (Amersham) under stringent conditions is 15 minutes. As a probe for screening a variant rpkA gene, a partial sequence of the nucleotide sequence disclosed in Table 1 can be used. Such a probe can be prepared by PCR using oligonucleotides based on the nucleotide sequence disclosed in Table 1 as primers, and a DNA fragment containing the nucleotide sequence disclosed in Table 1 as a template. When a DNA fragment having a length of about 300 bp is used as a probe, washing after the hybridization may be performed under the following conditions: 50° C., 2×SSC, and 0.1% SDS at least twice.

A DNA coding for fructose-6-phosphate phospholetolase can be obtained by similar procedures as described above using the nucleotide sequence disclosed in Table 2 as a probe.

It is known that some distribution bias of synonymous codons exists among the 61 amino acid codons found within the population of mRNA molecules, and the level of cognate tRNA appears directly proportional to the frequency of codon usage (see, for example, Kane, J. F., Curr. Opin. Biotechnol., 6(5), 494-500(1995)). From this point of view, it is easy to predict translational problems with abundant mRNA species which contain an excess of rare tRNA codons. Such a situation might arise after the initiation of transcription of a cloned heterologous gene in, for example, the E. coli host. Recent studies suggest clusters of AGG/AGA, CUA, AUA, CGA, or CCC codons may reduce both the quantity and quality of the synthesized protein. In addition, it is likely that an excess of any of these codons, even without clusters, could create translational problems. So, when a heterologous DNA coding for a protein of interest is transferred to the bacterium, it is desirable to replace rare codons by more frequently-used codons to avoid these translation problems. Frequency of codon usage in more than 11,000 organisms is presented in the “Codon usage database” (http://www.kazusa.or.jp/codon; Nakamura, Y. et al, Nucl. Acids Res., 28, 292 (2000)).

Furthermore, the gene encoding phosphoketolase of the present invention is not limited to a wild-type gene, but may be a mutant or artificially modified gene having modifications in the nucleotide sequences shown in Table 1 and 2. The encoded proteins may include substitutions, deletions, or insertions, of one or several amino acid residues at one or more positions in the amino acid sequences shown in Table 1 and 2, so long as the function of the encoded phosphoketolase protein is maintained. Although the number of “several” amino acid residues referred to herein differs depending on positions in the three-dimensional structure or types of amino acid residues, it may be 2 to 20, preferably 2 to 10, more preferably 2 to 5. Such substitutions of amino acids include functionally neutral sense mutations, and are preferably conservative substitutions. In the case of aromatic amino acids, conservative substitutions include phe, trp, and tyr interchangeably for each other. In the case of hydrophobic amino acids, conservative substitutions include leu, ile, and val interchangeably for each other. In the case of polar amino acids, conservative substitutions include gln and asn interchangeably for each other. In the case of basic amino acids, conservative substitutions include arg, lys, and his interchangeably for each other. In the case of acidic amino acids, conservative substitutions include asp and glu interchangeably for each other. In the case of hydroxyl group-containing amino acids, conservative substitutions include ser and thr interchangeably for each other. The conservative substitutions also include substitution of ser or thr for ala, substitution of gln, his or lys for arg, substitution of glu, gln, lys, his or asp for asn, substitution of asn, glu or gln for asp, substitution of ser or ala for cys, substitution of asn, glu, lys, his, asp or arg for gln, substitution of gly, asn, gln, lys or asp for glu, substitution of pro for gly, substitution of asn, lys, gln, arg or tyr for his, substitution of leu, met, val or phe for ile, substitution of ile, met, val or phe for leu, substitution of asn, glu, gln, his or arg for lys, substitution of ile, leu, val or phe for met, substitution of trp, tyr, met, ile or leu for phe, substitution of thr or ala for ser, substitution of ser or ala for thr, substitution of phe or tyr for trp, substitution of his, phe or trp for tyr and substitution of met, ile or leu for val. Mutations causing such substitution, deletion, or insertion of one or several amino acids as described above also include naturally occurring mutations arising from individual differences, and differences in species of microorganisms that harbor the phosphoketolase gene (mutant or variant). The region to be substituted can be selected by searching for the preserved region (motif) in PKT with MOTIF and Pfam.

Such genes can be obtained by modifying a nucleotide sequence shown in Table 1 or 2, for example, by site-specific mutagenesis, so that one or more substitutions, deletions, or insertions are introduced at a specific site of the protein encoded by the gene.

Furthermore, such genes can also be obtained by conventional mutagenesis treatments such as those mentioned below. Examples of mutagenesis treatments include treating a gene having a nucleotide sequence shown in Table 1 or 2 or a nucleotide sequence of nucleotide numbers 49 to 948 in SEQ ID NO: 1 in vitro with hydroxylamine, and treating a bacterium such as an Escherichia bacterium harboring the gene with ultraviolet ray irradiation or a mutagenesis agent used in a typical mutation treatments such as N-methyl-N′-nitro-N-nitrosoguanidine (NTG) or EMS (ethyl methanesulfonate).

Expression of the phospoketolase gene (hereinafter, referred to as pkt gene, including xpk or xfp gene) may be enhanced by, for example, increasing the copy number of the pkt gene in cells using genetic recombination techniques. For example, a recombinant DNA can be prepared by ligating a gene fragment containing the pkt gene to a vector, preferably a multi-copy vector, which can replicate in the host bacterium, and introducing the resulting vector into the host bacterium.

When the xfp gene of Bifidobacterium animalis is used, it may be obtained by, for example, the PCR method (polymerase chain reaction, refer to White, T. J. et al., Trends Genet., 5, 185 (1989)) using primers designed based on the nucleotide sequence of the xfp gene of Bifidobacterium animalis (SEQ ID NO: 9 or 11), for example, primers each having a nucleotide sequence of SEQ ID NO: 13 or 14, and using a chromosomal DNA of Bifidobacterium animalis as a template. The pkt gene from other microorganisms may also be used, and can be obtained from their chromosomal DNA or chromosomal DNA library by PCR using oligonucleotide primers designed based on a sequence of their pkt gene or a homologous sequence thereof, or by hybridization using an oligonucleotide probe prepared based on such sequence information as shown in Table 1 or 2. A chromosomal DNA to be used as a DNA donor can be prepared from a microorganism, for example, by the method of Saito and Miura (refer to H. Saito and K. Miura, Biochem. Biophys. Acta, 72, 619 (1963), Text for Bioengineering Experiments, Edited by the Society for Bioscience and Bioengineering, Japan, pp. 97-98, Baifukan, 1992).

Then, the pkt gene is ligated to a vector DNA operable in the host bacterium to prepare a recombinant DNA. Preferably, vectors autonomously replicable in the host bacterium are used.

Examples of vectors autonomously replicable in Escherichia coli include pUC19, pUC18, pHSG299, pHSG399, pHSG398, pACYC184, (pHSG and pACYC are available from Takara Bio), RSF1010, pBR322, pMW118, pMW219 (pMW is available from Nippon Gene), and so forth. Phage vectors such as 11059, IBF101, M13mp9 may also be used.

Examples of vectors which are autonomously replicable in Coryneform bacteria include pAM330 (EP 0077548B), pHM1519 (EP 0078537), pGA1 (EP10977998), and pYM2(U.S. Pat. No. 6,905,819)

Moreover, a so-called shuttle vector autonomously replicable in both Escherichia coli and Coryneform bacteria may also be used, for example, pVK9, pVK7 (U.S. 2003-0175912), pSFK6 (JP2000-262288A), and pHK4 (JP5-007491A).

The pVK9 is an E. coli -coryneform shuttle vector obtained by excising a BamHI-KpnI fragment containing the replicable origin from pHK4 (JP-A-5-007491) and introducing it into the AvaII site of pHSG299 (Product of Takara-Bio).

In order to prepare a recombinant DNA by ligating the pkt gene and any of the vectors mentioned above, the vector and a fragment containing the pkt gene are ligated, usually by using a ligase such as a T4 DNA ligase.

To introduce a recombinant DNA prepared as described above into a bacterium, any known transformation method reported so far can be employed. For example, a method of treating recipient cells with calcium chloride so as to increase the permeability of DNA, which has been reported for Escherichia coli (Mandel, M. and Higa, A., J. Mol. Biol., 53, 159 (1970)), and a method of using competent cells prepared from growing cells to introduce a DNA, which has been reported for Bacillus subtilis (Duncan, C. H., Wilson, G. A. and Young, F. E., Gene, 1, 153 (1977)) can be employed. In addition to these methods, a method of introducing a recombinant DNA into protoplast- or spheroplast-like recipient cells, which have been reported to be applicable to Bacillus subtilis, actinomycetes, and yeasts (Chang, S. and Choen, S. N., Molec. Gen. Genet., 168, 111 (1979); Bibb, M. J., Ward, J. M. and Hopwood, O. A., Nature, 274, 398 (1978); Hinnen, A., Hicks, J. B. and Fink, G. R., Proc. Natl. Sci., USA, 75, 1929 (1978)), can be employed. In addition, transformation of Coryneform bacteria can also be performed by the electric pulse method (Sugimoto et al., JP2-207791A).

The pkt gene can be introduced by integrating a single copy or multiple copies of the gene into a chromosomal DNA of a bacterium. Furthermore, the copy number of the pkt gene can also be increased by integrating multiple copies of the gene into a chromosomal DNA of a bacterium. In order to integrate one or more copies of the pkt gene on a chromosomal DNA of a bacterium, homologous recombination can be performed by targeting a sequence which exists in multiple copies on a chromosomal DNA. Repetitive DNA and inverted repeats at an end of a transposon can be used as a sequence which exists on a chromosomal DNA in multiple copies. Alternatively, as disclosed in JP2-109985A, it is also possible to incorporate the pkt gene into a transposon, and allow it to be transferred so that the gene is integrated into the chromosomal DNA. Furthermore, enhancement of gene expression may be performed by transposition such as Mu integration. For example, one round of Mu integration allows for introduction into a bacterial chromosome of up to 3 copies of the gene. Transposons such as Mu, Tn10 and Tn5 may be used. Integration of the pkt gene into the chromosome can be confirmed by Southern hybridization using a probe having a partial sequence of the pkt gene.

Enhancing expression of the pkt gene can also be attained by either replacing an expression regulatory sequence, including a promoter of the pkt gene, on a chromosomal DNA or on a plasmid with a stronger one, as described in WO00/18935, amplifying a regulatory factor that increases expression of the pkt gene, or deleting or attenuating a regulatory factor that reduces expression of the pkt gene. Moreover, it is also possible to introduce several nucleotide substitutions into a promoter region of the pkt gene so that the promoter is more potent. A method for evaluating potency of promoter and examples of potent promoters are disclosed in Goldstein et al. (Prokaryotic promoters in biotechnology. Biotechnol. Annu. Rev., 1995, 1, 105-128). Furthermore, since it is known that a spacer sequence between the ribosome binding site (RBS) and translation initiation codon, especially, several nucleotides just upstream of the initiation codon, has a great influence on translation efficiency, this sequence may be modified. Expression regulatory sequences of pkt gene may be identified using a vector for promoter identification or genetic analysis software such as GENETYX.

Example of such strong promoters include the lac promoter, trp promoter, trc promoter, tac promoter, P_(R) promoter, p_(L) promoter of lambda phage, tet promoter, amyE promoter, spac promoter, and so forth. An example of a strong promoter for corynebacterium includes the PS2 promoter (Appl Environ Microbiol. January 2003; 69(1):358-66; Mol Microbiol. July 1993; 9(1):97-109; WO93/03158), trc promoter, tac promoter, lacUV5 promoter, araBAD promoter. Promoter strength is defined as the frequency of acts of RNA synthesis initiation. Methods for evaluating the strength of a promoter are described by, for example, Deuschle U., Kammerer W., Gentz R., Bujard H. (Promoters in Escherichia coli: a hierarchy of in vivo strength indicates alternate structures. EMBO J., 5, 2987-2994 (1986)).

The expression of the pkt gene is enhanced by such substitution or modification of a promoter. The substitution of an expression regulatory sequence can also be attained by, for example, using a temperature-sensitive plasmid. Examples of a temperature-sensitive plasmid for Coryneform bacteria include p48K and pSFKT2 (JP2000-262288A), pHSC4 (refer to France Patent Laid-open Publication No. 2667875, 1992 and JP5-7491A), pBS5T, and so forth. These plasmids can autonomously replicate at least at a temperature of 25° C., but cannot autonomously replicate at a temperature of 37° C. in Coryneform bacteria. Modifying the expression regulatory sequence may be combined with increasing the copy number of the pkt gene.

The method of using levansucrase, which is lethal to corynebacterium, as a marker of homologous recombination is also suitable. The sacB gene encoding levansucrase can be used to select the strain in which the vector is deleted from chromosome effectively. (Schafer, A. et al. Gene 145 (1994)69-73). The sacB gene and homologous gene of sacB described below can be used.

-   Bacillus subillus: sacB GenBank Accession Number X02730 (SEQ ID NO:     40) -   Bacillus amyloliqufaciens: sacB GenBank Accession Number X52988 -   Zymomonas mobilis: sacB GenBank Accession Number L33402 -   Bacillus stearothermophilus: surB GenBank Accession Number U34874 -   Lactobacillus sanfranciscensis: frfA GenBank Accession Number     AJ508391 -   Acetobacter xylinus: 1sxA GenBank Accession Number AB034152 -   Gluconacetobacter diazotrophicus: 1sdA GenBank Accession Number     L41732

Preparation of plasmid DNA, digestion and ligation of DNA, transformation, selection of an oligonucleotide as a primer and the like can be performed by ordinary methods well-known by those skilled in the art. Such methods are described, for example, in Sambrook, J., Fritsch, E. F., and Maniatis, T., “Molecular Cloning A Laboratory Manual, Second Edition”, Cold Spring Harbor Laboratory Press (1989).

The bacteria of the present invention include not only a bacterium which does not inherently have an activity of D-xylulose-5-phosphate phosphoketolase and fructose-6-phosphate phosphoketolase, but also a bacterium which inherently has an activity of these proteins and has been modified so that the activity is enhanced. The former is preferably used.

Specific examples of the bacterium used in the present invention include bacteria belonging to Enterobacteriaceae family such as the genus Escherichia, Pantoea, Coryneform bacterium such as Corynebacterium glutamicum, Bacillus bacterium such as Bacillus subtilis, and so forth. However, the bacterium of the present invention is not limited to these examples.

The Enterobacteriaceae family includes bacteria belonging to the genera Enterobacter, Erwinia, Escherichia, Klebsiella, Pantoea, Providencia, Salmonella, Serratia, Shigella, Morganella etc. Specifically, those classified into the Enterobacteriaceae according to the taxonomy used in the NCBI (National Center for Biotechnology Information) database (http://www.ncbi.nlm.nih.gov/htbin-post/Taxonomy/wgetorg?mode=Tree&id=1236&lvl=3&keep=1 &srchmode=1&unlock) can be used. A bacterium belonging to the genus of Escherichia or Pantoea is preferred.

Escherichia bacteria reported in Neidhardt et al. (Neidhardt, F. C. et al., Escherichia coli and Salmonella Typhimurium, American Society for Microbiology, Washington D.C., 1208, Table 1), such as Escherichia coli, can be utilized. Examples of wild-type strains of Escherichia coli include, but are not limited to, the K12 strain and derivatives thereof, MG1655 strain (ATCC No. 47076), and W3110 strain (ATCC No. 27325). These strains are available from the American Type Culture Collection (ATCC, Address: P.O. Box 1549, Manassas, Va. 20108, United States of America).

The term “a bacterium belonging to the genus Pantoea” means that the bacterium is classified as the genus Pantoea according to the classification known to a person skilled in the art of microbiology, and also includes some species of Enterobacter agglomerans recently re-classified into Pantoea agglomerans, Pantoea ananatis, Pantoea stewartii or the like, based on nucleotide sequence analysis of 16S rRNA etc.

In the present invention, coryneform bacteria include bacteria classified as Coryneform bacterium according to the classification known to a person skilled in the art of microbiology, bacteria which were hitherto classified into the genus Brevibacterium, but have now been re-classified into the genus Corynebacterium (Int. J. Syst. Bacteriol., 41, 255 (1991)), and also bacteria belonging to the genus Brevibacterium, which is a close relative of the genus Corynebacterium. Examples of such Coryneform bacteria are listed below.

-   Corynebacterium acetoacidophilum -   Corynebacterium acetoglutamicum -   Corynebacterium alkanolyticum -   Corynebacterium callunae -   Corynebacterium glutamicum -   Corynebacterium lilium -   Corynebacterium melassecola -   Corynebacterium thermoaminogenes -   Corynebacterium herculis -   Brevibacterium divaricatum -   Brevibacterium flavum -   Brevibacterium immariophilum -   Brevibacterium lactofermentum -   Brevibacterium roseum -   Brevibacterium saccharolyticum -   Brevibacterium thiogenitalis -   Corynebacterium ammoniagenes -   Brevibacterium album -   Brevibacterium cerinum -   Microbacterium ammoniaphilum

Specifically, the following strains are encompassed:

-   Corynebacterium acetoacidophilum ATCC 13870 -   Corynebacterium acetoglutamicum ATCC 15806 -   Corynebacterium alkanolyticum ATCC 21511 -   Corynebacterium callunae ATCC 15991 -   Corynebacterium glutamicum ATCC 13020, 13032, 13060 -   Corynebacterium lilium ATCC 15990 -   Corynebacterium melassecola ATCC 17965 -   Corynebacterium thermoaminogenes AJ12340 (FERM BP-1539) -   Corynebacterium herculis ATCC 13868 -   Brevibacterium divaricatum ATCC 14020 -   Brevibacterium flavum ATCC 13826, ATCC 14067, AJ12418 (FERM BP-2205) -   Brevibacterium immariophilum ATCC 14068 -   Brevibacterium lactofermentum (Corynebacterium glutamicum) ATCC     13869 -   Brevibacterium roseum ATCC 13825 -   Brevibacterium saccharolyticum ATCC 14066 -   Brevibacterium thiogenitalis ATCC 19240 -   Brevibacterium ammoniagenes ATCC 6871, ATCC 6872 -   Brevibacterium album ATCC 15111 -   Brevibacterium cerium ATCC 15112 -   Microbacterium ammoniaphilum ATCC 15354

These strains can be obtained from, for example, the American Type Culture Collection (ATCC, Address: P.O. Box 1549, Manassas, Va. 20108, United States of America). Each strain is assigned a registration number, and one can request a provision of each strain by its registration number. The registration number for each strain is indicated in the catalog of the American Type Culture Collection. The AJ12340 strain was deposited on Oct. 27, 1987 at the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Ministry of International Trade and Industry (currently, the independent administrative agency, National Institute of Advanced Industrial Science and Technology, International Patent Organism Depositary (Tsukuba Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, Japan, postal code: 305-5466)) under the provisions of the Budapest Treaty, and received an accession number of FERM BP-1539. The AJ12418 strain was deposited on Jan. 5, 1989 at the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Ministry of International Trade and Industry under the provisions of the Budapest Treaty and received an accession number of FERM BP-2205.

The term “Bacillus bacterium” means that the bacterium classified as the genus Bacillus according to the classification known to a person skilled in the art of microbiology. In the present invention, the Bacillus bacterium include, but are not limited to, the Bacillus subtilis 168 Marburg strain (ATCC 6051), Bacillus subtilis PY79 strain (Plasmid, 1984, 12, 1-9) and so forth, and examples of Bacillus amyloliquefaciens include, but are not limited to, Bacillus amyloliquefaciens T strain (ATCC 23842), Bacillus amyloliquefaciens N strain (ATCC 23845) and so forth.

The bacterium of the present invention can be obtained by introducing the aforementioned DNA (pkt gene) into a bacterium which already has an ability to produce a useful metabolite. Alternatively, the bacterium of present invention can be obtained by imparting the ability to produce a useful metabolite to the bacterium into which the pkt gene has been introduced. In the latter case, introducing the pkt gene and imparting the ability to produce the useful metabolite may be performed in any order.

The phrase “a bacterium having an ability to produce a useful metabolite” means a bacterium, which has an ability to produce and cause accumulation of the useful metabolite in a medium when the bacterium of the present invention is cultured in the medium. The ability to produce the metabolite may be imparted or enhanced by breeding including mutagenesis treatment and genetic modification. The phrase “bacterium has ability to produce useful metabolite” means that the bacterium is able to produce and cause accumulation of the metabolite in a medium in an amount larger than a wild-type strain or unmutated strain. The phrase “bacterium has ability to produce a useful metabolite” as used herein also means a bacterium that is able to produce and cause accumulation in a medium of an amount not less than 0.5 g/L, more preferably not less than 1.0 g/L of the target metabolite.

The phrase “useful metabolite” means a primary metabolite such as an L-amino acid, organic acid, vitamin, saccharide, and/or intermediate of Embden-Meyerhof, pentose phosphate (pentose-P) pathways, Entner-Doudoroff pathway, citrate cycle, and amino acid biosynthesis. The L-amino acid to be produced in the present invention is not particular limited, and includes L-lysine, L-arginine, L-ornithine, L-histidine, L-citrulline, L-isoleucine, L-alanine, L-valine, L-leucine, and L-glycine, L-threonine, L-serine, L-proline, L-phenylalanine, L-tyrosine, and L-tryptophan, L-cysteine, L-cystine, L-methionine, L-ornithine, L-glutamic acid, L-aspartic acid, L-glutamine, and L-asparagine. More preferably, the bacterium of present invention is a bacterium having an ability to produce a metabolite derived from acetyl-CoA. Such metabolite is selected from the group consisting of L-glutamic acid, L-glutamine, L-proline, L-arginine, L-leucine, L-cysteine, succinate, and polyhydroxybutyrate.

Hereinafter, examples of strains imparted with an ability to produce useful metabolites and methods of imparting such ability will be explained in detail.

Hereinafter, methods for imparting an ability to produce useful metablites to a parent strain as mentioned above will be explained.

In order to impart an ability to produce useful metabolites, methods conventionally used for breeding an useful metabolite-producing bacterium belonging to the genus Escherichia or Coryneform bacterium and so forth can be used. For example, methods for obtaining an auxotrophic mutant strain, analogue-resistant strain, or metabolic regulation mutant strain having an ability to produce useful metabolites, and methods for creating a recombinant strain having enhanced activity of an useful metabolites biosynthetic enzyme can be used (“Amino Acid Fermentation”, the Japan Scientific Societies Press [Gakkai Shuppan Center], 1st Edition, published on May 30, 1986, pp. 77-100). When breeding useful metabolite-producing bacteria using these methods, one or more properties, including auxotrophy, analogue-resistance, and metabolic regulation mutation, may be imparted.

When a recombinant strain is created, the activity of single or multiple useful metabolites-biosynthetic enzymes may be enhanced. Furthermore, methods imparting properties of auxotrophy, analogue resistance, and metabolic regulation mutation may be combined with methods for enhancing an activity of an useful metabolite-biosynthetic enzyme.

An auxotrophic mutant strain, useful metabolites such as an L-amino acid analogue-resistant strain, or a metabolic regulation-mutated strain having an useful metabolite-producing ability can be obtained by subjecting a parent or wild-type strain to a typical mutagenesis treatment such as X-ray or ultraviolet ray irradiation, treatment with a mutagenesis agent such as N-methyl-N′-nitro-N-nitrosoguanidine (NTG). Then, an auxotrophic strain, analogue-resistant strain, or metabolic regulation mutant strain which has a useful metabolite-producing ability may be selected from the mutated strains.

Methods of imparting L-glutamic acid-producing ability to the bacteria as described above include, for example, modifying the bacteria so that expression of a gene encoding an enzyme involved in L-glutamic acid biosynthesis is enhanced and/or overexpressed. Examples of enzymes involved in the L-glutamic acid biosynthesis include glutamate dehydrogenase (also referred to as “GDH” hereinafter), glutamine synthetase, glutamate synthase, isocitrate dehydrogenase, aconitate hydratase, citrate synthase (also referred to as “CS” hereinafter), phosphoenolpyruvate carboxylase (also referred to as “PEPC” hereinafter), pyruvate carboxylase, pyruvate dehydrogenase, pyruvate kinase, phosphoenolpyruvate synthase, enolase, phosphoglyceromutase, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase, triose phosphate isomerase, fructose bisphosphate aldolase, phosphofructokinase, glucose phosphate isomerase, and so forth. Of these enzymes, it is preferable that the activity of one or more of CS, PEPC, and GDH is/are enhanced, and it is more preferable that activities of all three of these enzymes are enhanced.

Examples of bacteria modified by the method as described above so that expression of the CS gene, PEPC gene, and/or GDH gene are enhanced include the bacteria disclosed in U.S. Pat. No. 6,197,559, U.S. Pat. No. 6,331,419, and European Patent Publications No.0999282 and No.1078989.

The modification of a bacterium to impart L-glutamic acid-producing ability may be performed by reducing or inactivating the activity of an enzyme that catalyzes a reaction branching from the L-glutamic acid biosynthetic pathway, and producing a compound other than L-glutamic acid. Examples of enzymes which catalyze a reaction branching from the L-glutamic acid biosynthetic pathway and producing a compound other than L-glutamic acid include 2-oxoglutarate dehydrogenase, isocitrate lyase, glutamate decarboxylase, 1-pyrroline dehydrogenase, and so forth. Of these enzymes, it is preferable to reduce or eliminate the activity of 2-oxoglutarate dehydrogenase.

To reduce or inactivate the activities of the aforementioned enzymes, mutations for reducing or inactivating intracellular activities of the enzymes can be introduced by usual mutagenesis treatment or genetic engineering. Examples of mutagenesis treatment include irradiation by X-rays or ultraviolet rays, treament with a mutagenesis agent such as N-methyl-N′-nitro-N-nitrosoguanidine, and so forth. Examples of methods for reducing or eliminating the intracellular activity of an enzyme include mutating or deleting a gene encoding the enzyme in the cells of a microorganism so that intracellular activity is reduced or eliminated as compared to a non-mutated strain. Examples of methods for mutating or deleting a gene include modification of expression regulatory sequences such as promoters and Shine-Dalgarno (SD) sequences, introduction of mis-sense mutations, non-sense mutations, or frame-shift mutations into an open reading frame, and deletion of a portion of the gene (J Biol Chem. 1997 272(13):8611-7). A mutated gene can be introduced into a microorganism by using a homologous recombination technique in which a wild-type gene on a chromosome is replaced with the mutated gene, or by using a transposon or IS factor. Homologous recombination techniques include methods using linear DNA, a temperature-sensitive plasmid, and non-replicable plasmid. These methods are described in Proc Natl Acad Sci U S A. Jun. 6, 2000; 97(12):6640-5., U.S. Pat. No. 6,303,383, JP05-007491A, and the like.

Intracellular activity of the target enzyme and the degree of decrease in the activity can be confirmed by measuring the enzyme activity using a cell extract or a purified fraction thereof obtained from a candidate strain and comparing it with the activity of a wild-type or non-modified strain. For example, the 2-oxoglutarate dehydrogenase activity can be measured by the method of Reed et al. (Reed L. J. and Mukherjee B. B., Methods in Enzymology, 13, pp. 55-61, 1969). Examples of bacteria belonging to the genus Escherichia deficient in α-ketoglutarate dehydrogenase activity or having reduced α-ketoglutarate dehydrogenase activity and methods for obtaining them are described in U.S. Pat. Nos. 5,378,616 and 5,573,945. Specifically, these strains include the following:

-   E. coli W3110sucA::Kmr -   E. coli AJ12624 (FERM BP-3853) -   E. coli AJ12628 (FERM BP-3854) -   E. coli AJ12949 (FERM BP-4881)

E. coli W3110sucA::Km^(r) was obtained by disrupting the α-ketoglutarate dehydrogenase gene (hereinafter referred to as “sucA gene”) in E. coli W3110. This strain is completely deficient in the α-ketoglutarate dehydrogenase.

Coryneform bacteria in which sucA gene is disrupted using homologous recombination are explained in detail in WO95/34672 and U.S. Pat. No. 5,977,331.

Other examples of L-glutamic acid-producing bacterium include those which belong to the genus Escherichia and have resistance to an aspartic acid antimetabolite. These strains may also be deficient in alpha-ketoglutaric acid dehydrogenase activity and include, for example, strain AF13199 (FERM BP-5807) (U.S. Pat. No. 5,908,768), or strain FFRM P-12379, which is additionally modified to have a lowered L-glutamic acid-decomposing ability (U.S. Pat. No. 5,393,671); E. coli strain AJ13138 (FERM BP-5565) (U.S. Pat. No. 6,110,714) and the like.

Examples of L-glutamic acid-producing bacteria belonging to the genus Pantoea, include mutant strains deficient in α-ketoglutarate dehydrogenase activity or with decreased α-ketoglutarate dehydrogenase activity, and can be obtained as described above. Such strains include Pantoea ananatis AJ13356 or AJ13601 (U.S. Pat. No. 6,331,419). Pantoea ananatis AJ13356 was deposited at the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Ministry of International Trade and Industry (currently, National Institute of Advanced Industrial Science and Technology, International Patent Organism Depositary, Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan) on Feb. 19, 1998 and received an accession number of FERM P-16645. It was then converted to an international deposit under the provisions of Budapest Treaty on Jan. 11, 1999 and received an accession number of FERM BP-6615. Pantoea ananatis AJ13601 was deposited at the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Ministry of International Trade and Industry (currently, National Institute of Advanced Industrial Science and Technology, International Patent Organism Depositary, Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan) on Aug. 18, 1999 and received an accession number of FERM P-17516. It was then converted to an international deposit under the provisions of Budapest Treaty on Jul. 6, 2000 and received an accession number of FERM BP-7207. The Pantoea ananatis AJ13356 and AJ13601 are deficient in α-KGDH activity as a result of disruption of the gene encoding the E1 subunit of αKGDH (sucA gene). The above strains were identified as Enterobacter agglomerans when they were isolated and deposited as the Enterobacter agglomerans AJ13356 strain, and AJ13601 strain, respectively. However, they were recently re-classified as Pantoea ananatis on the basis of nucleotide sequencing of 16S rRNA and so forth. Although AJ13356, AJ13601, and their parent strain AJ13355 (FERM BP-6614), were all deposited at the aforementioned depository as Enterobacter agglomerans, they are described as Pantoea ananatis for the purposes of this specification.

Further examples of Coryneform bacteria having L-glutamic acid-producing ability include an organic acid analogue-resistant mutant, and escletin-resistant mutant (JP56-1889A, JP56140895A, JP5702689A, JP88994A).

Examples of analogue-resistant L-glutamic acid-producing Coryneform bacterium include the following strains:

-   Brevibacterium flavum AJ11355 (FERM P-5007, JP56-1889A) -   Brevibacterium glutamicum AJ11368 (FERM P-P-5020, JP56-1889A) -   Brevibacterium flavum AJ11217 (FERM P-4318, JP57-2689A) -   Brevibacterium flavum AJ11218 (FERM-P 4319, JP57-2689A) -   Brevibacterium flavum AJ11564 (FERM P-5472, JP56-140895A) -   Brevibacterium flavum AJ11439 (FERM P-5316, JP56-35981A) -   Corynebacterium glutamicum H7684 (FERM BP-3004, JP56-151495A)

Examples of L-glutamine producing bacterium which can be used as a parent strain to be modified to have increased activity of the D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase include an L-glutamine-producing coryneform bacterium which is modified to have enhanced glutamate dehydorogenase activity, glutamine synthetase activity (EP 1229121A), or to have decreased glutaminase activity (EP1424397A). Also, a bacterium belonging to the genus Escherichia harboring a mutant glutamine synthetase in which the tyrosine amino acid residue corresponding to position 397 in a wild-type glutamine synthetase is replaced with any amino acid residue can be used.

Methods of imparting 6-diazo-5-oxo-norleucine-resistance (JP3-232497A), purine analogue-resistance, methionine sulfoxide-resistance (JP61-202694A), α-Ketomalinic acid-resistance (JP56-151495) can be used to impart or enhance the L-glutamine producing ability by breeding. Specific examples of Coryneform bacteria having L-glutamine-producing ability include, but are not limited to:

-   Brevibacterium flavum AJ11573 (FERM P-5492, JP56-161495A) -   Brevibacterium flavum AJ11576 (FERM BP-10381, JP56-161495A) -   Brevibacterium flavum AJ12212 (FERM P-8123, JP61-202694A)

Examples of L-proline-producing bacterium useful as a parent strain to be modified to have increased activity of the D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase include an L-proline-producing bacterium belonging to the genus Escherichia which harbors γ-glutamyl kinase desensitized in feedback inhibition by L-proline, and/or in which the L-proline degradation system is destroyed. A method for breeding the bacterium having γ-glutamyl kinase desensitized in feedback inhibition by L-proline is exemplified by introducing a DNA coding for γ-glutamyl kinase desensitized in feedback inhibition by L-proline into cells (Dandekar, A. M., Uratsu, S. L., J. Bacteriol., 170, 12, 5943-5 (1988)). A method for destroying the L-proline degradation system is exemplified by introducing a mutation into a proline dehydrogenase gene so that the active proline dehydrogenase is not expressed. Also, a bacterium in which the L-proline degradation system is destroyed can be obtained by obtaining a strain deficient in L-proline-assimilating ability and selecting a strain which overproduces L-proline extracellularly by using L-proline auxotrophy as an index. The L-proline-producing bacteria belonging to the genus Escherichia include E. coli strains NRRL B-12403 and NRRL B-12404 (GB Patent 2075056), VKPM B-8012 (U.S. patent Laid-open Publication 2002-0058315), and plasmid mutants described in DE Patent 3127361, plasmid mutants described by Bloom F. R. et al (The 15^(th) Miami winter symposium, 1983, p. 34) and the like may be used for obtaining such strains.

Examples of L-arginine-producing bacterium which can be used as a parent strain to be modified to have increased activity of the D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase include L-arginine-producing bacteria belonging to the genus Escherichia, for example, E. coli strain 237 (VKPM B-7925) and its derivative strains having mutant N-acetylglutamate synthase (U.S. patent Laid-open Publication 2002-0034793), an arginine-producing strain into which an argA gene encoding N-acetylglutamate synthetase is introduced (JP57-5693A), and the like.

Another example of an L-arginine-producing bacterium is a strain which has amplified genes coding for L-arginine-biosynthetic enzymes such as N-acetylglutamate synthase (argA), N-acetylglutamyl phosphate reductase (argC), ornithine acetyltransferase (argJ), N-acetylglutamate kinase (argB), acetylornithine transaminase (argD), acetylornithine deacetylase (argE), ornithine carbamoyltransferase (argF), argininosuccinate synthase (argG), argininosuccinate lyase (argH), and carbamoyl-phosphate synthase (carAB). The names of the genes encoding these enzymes are given in parentheses after the names of the enzymes, respectively.

Examples of L-leucine-producing bacterium which can be used as a parent strain to be modified to have increased activity of the D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase include the L-leucine-producing bacteria belonging to the genus Escherichia, such as E. coli strains H-9068 (ATCC 21530), H-9070 (FERM BP-4704), and H-9072 (FERM BP-4706) resistant to 4-azaleucine or 5,5,5-trifluoroleucine (U.S. Pat. No. 5,744,331), E. coli strains in which feedback inhibition of isopropylmalate synthase by L-leucine is desensitized (EP1067191B), E. coli strain AJ11478 resistant to β-2-thienylalanine, and β-hydroxyleucine (U.S. Pat. No. 5,763,231), E. coli strain 57 (VKPM B-7386, Russian patent No.2140450), and the like.

Examples of L-cysteine-producing bacterium which can be used as a parent strain to be modified to have increased activity of the D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase include the L-cysteine-producing bacterium belonging to the genus Escherichia, such as the E. coli strain JM15 which is transformed with different cysE alleles coding for feedback-resistant serine acetyltransferases (U.S. Pat. No. 6,218,168); E. coli strain W3110 over-expressing a gene encoding a protein suitable for secreting cytotoxic substances (U.S. Pat. No. 5,972,663); E. coli strains having lowered cysteine desulfohydrase activity (JP11-155571A2); E. coli strain W3110 using increased activity of positive transcriptional regulator for cysteine regulon encoded by cysB gene (WO01/27307A1), and the like.

Examples of succinate-producing bacterium which can be used as a parent strain to be modified to have increased activity of the D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase include the succinate-producing bacterium belonging to the genus Escherichia and Coryneform bacterium such as Brevibacterium flavum MJ233Δ1dh strain (JP11-206385) and Brevibacterium flavum MJ233/pPCPYC strain (WO01/27258).

Examples of an L-lysine-producing bacterium which can be used as a parent strain to be modified to have increased activity of the D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase include S-(2-aminoethyl)cysteine (hereinafter “AEC”)-resistant mutant strains of Brevibacterium lactofermentum AJ11082 (NRRL B-11470) (described in JP56-1914B, JP56-1915B, JP57-14157B, JP57-14158B, JP57-30474B, JP58-10075B, JP59-4993B, JP61-35840B, JP62-24074B, JP62-36673B, JP5-11958B, JP7-112437B, and JP7-112438B), mutant strains auxotrophic for an amino acid such as L-homoserine (JP48-28078B and JP56-6499B), mutant strains resistant to AEC and auxtrophic for an amino acid such as L-leucine, L-homoserine, L-proline, L-serine, L-arginine, L-alanine, and L-valine (U.S. Pat. Nos. 3,708,395 and 3,825,472), L-lysine-producing mutant strains resistant to DL-α-amino-ε-caprolactam, α-amino-lauryllactam, aspartic acid analogues, sulfa drugs, quinoid, and N-lauroylleucine, L-lysine-producing mutant strains resistant to oxaloacetate decarboxylase inhibitor or a respiratory tract enzyme inhibitor (JP50-53588A, JP50-31093A, JP52-102498A, JP53-9394A, JP53-86089A, JP55-9783A, JP55-9759A, JP56-32995A, JP56-39778A, JP53-43591B and JP53-1833B), L-lysine-producing mutant strains auxotrophic for inositol or acetic acid (JP55-9784A and JP56-8692A), L-lysine-producing mutant strains that are susceptible to fluoropyruvic acid at a temperature of 34° C. or higher (JP55-9783A and JP53-86090A), L-lysine-producing mutant strains of Brevibacterium or Corynebacterium bacteria resistant to ethylene glycol (U.S. Pat. No. 4,411,997), and so forth.

Further examples of L-lysine-producing bacteria which can be used as a parent strain to be modified to have increased activity of the D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase include S-(2-aminoethyl)cysteine (hereinafter “AEC”)-resistant mutant strains of Escherichia coli WC196 (WO96/17930). The WC196 strain was designated as Escherichia coli AJ13069, and deposited at the independent administrative agency, National Institute of Advanced Industrial Science and Technology, International Patent Organism Depositary (Tsukuba Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, Japan, postal code: 305-8566) on Dec. 6, 1994 and received an accession number of FERM P-14690. Then, the deposit was converted to an international deposit under the provisions of the Budapest Treaty on Sep. 29, 1995, and received an accession number of FERM BP-5252.

Examples of microorganisms having an L-threonine-producing ability include a 6-dimethylaminopurine-resistant mutant (JP5-304969A), a strain in which a gene for a threonine biosynthetic enzyme having a mutation which enhances the enzymatic activity is amplified with a plasmid (JP1-29559B, JP05-227977A), a strain in which the threonine operon is amplified with a plasmid (JP2-109985A), a strain in which a gene encoding pyruvate carboxylase and a gene encoding nicotinamide nucleotide transhydrogenase are amplified (JP2002-51787A), and so forth.

The Escherichia coli VKPM B-3996 strain (U.S. Pat. No. 5,175,107) may also be used as an L-threonine-producing strain. VKPM B-3996 was deposited at the Russian National Collection of Industrial Microorganisms (VKPM), GNII Genetika with a registration number of VKPM B-3996 on Nov. 19, 1987. The VKPM B-3996 strain harbors plasmid pVIC40 (International Patent Publication WO90/04636), which is obtained by inserting a threonine operon (thrABC) into plasmid pAYC32 which has a streptomycin-resistance marker gene (Chistorerdov, A. Y., Tsygankov, Y. D., Plasmid, 1986, 16, 161-167). Aspartokinase I-homoserine dehydrogenase I encoded by a mutant thrA gene contained in pVIC40 is released from feedback inhibition by L-threonine.

The Escherichia coli VKPM B-5318 strain (EP 0593792B) may also be used as an L-threonine-producing strain. VKPM B-5318 was deposited at the Russian National Collection of Industrial Microorganisms (VKPM), GNII Genetika with a registration number of VKPM B-5318 on Nov. 19, 1987. The VKPM B-5318 strain is autotrophic to L-isoleucine, and the threonine operon encoding the threonine biosynthesis enzyme is located downstream of the C1 temparature-sensitive represser, the PR-promoter, and the N-terminus end of the Cro protein derived from λphage. Moreover, this strain contains plasmid DNA which was constructed so that expression of the threonine biosynthesis gene is regulated by the promoter and represser from λphage.

Furthermore, the Escherichia coli MG442 strain (U.S. Pat. No. 4,278,765) may also be used as an L-threonine-producing strain. The MG442 strain was deposited at the Russian National Collection of Industrial Microorganisms (VKPM) as CMIMB-1628.

Examples of Escherichia bacteria having an ability to produce L-phenylalanine include Escherichia coli AJ12739 (tyrA::Tn10, TyrR; VKPM B-8197) in which the tyrA and tyrR genes are disrupted, Escherichia coli HW1089 in which a mutated pheA has been introduced, (U.S. Pat. No. 5,354,672), and an Escherichia coli strain in which the yddG and yedA genes have been amplified (WO 03/044192). Examples of Coryneform bacteria having an ability to produce L-phenylalanine include a strain which is auxotrophic for tyrosine and resistant to L-phenylalanyl-L-tyrosine (JP5-49489A).

Bacteria having an ability to produce L-tryptophan can be obtained by enhancing the activities of the L-tryptophan-biosynthetic enzymes including phosphoglycerate dehydrogenase and anthranilate synthase. These enzymes may be resistant to feedback inhibition by L-tryptophan or L-serine. For example, a bacterium having these feedback-resistant enzymes can be obtained by introducing plasmid pGH5, which contains a mutant serA gene encoding L-tryptophan-resistant phosphoglycerate dehydrogenase, into Escherichia coli SV164 strain which harbors a gene encoding L-serine-resistant anthranilate synthase (WO94/0803 1).

Bacteria having an ability to produce L-tryptophan can also be obtained by enhancing the activities of L-tryptophan-biosynthetic enzymes encoded by the tryptophan operon. Such enzymes in the the L-tryptophan operon include tryptophan synthase and anthranilate synthase. Examples of these bacteria include an Escherichia coli strain in which the tryptophan operon containing a gene encoding L-serine-resistant anthranilate synthase is introduced (JP57-71397A, JP62-244382A, and U.S. Pat. No. 4,371,614).

In addition, examples of bacteria having an ability to produce L-tryptophan include Escherichia coli AGX17(pGX44) [NRRL B-12263] which is auxotrophic for L-phenylalanine and L-tyrosine, and AGX6(pGX50)aroP [NRRL B-12264] which harbors plasmid pGX50 containing the tryptophan operon (U.S. Pat. No. 4,371,614).

Examples of Escherichia bacteria having an ability to produce L-isoleucine include a mutant strain resistant to 6-dimethylaminopurine (JP5-304969A), a mutant strain resistant to L-isoleucinehydroxamate, thiaisoleucine, DL-ethionine, or argininehydroxamate (JP5-130882A), and a recombinant strain in which a gene encoding threonine deaminase and acetohydroxylic acid synthase have been amplified with a plasmid (JP2-458A, JP2-42988A and JP8-47397A).

Bacteria having an ability to produce L-valine can be obtained by enhancing the activities of L-valine biosynthetic enzymes including those encoded by the ilvGMEDA operon, especially acetohydroxylate synthase encoded by the ilvG gene (JP02-748418B). These enzymes may be resistant to feedback inhibition by L-valine.

Bacteria having an ability to produce L-valine include those which have decreased expression of the acetolactate synthase III gene (ilvIH gene).

Bacteria having an ability to produce L-valine may be resistant to amino acid analogues. Examples of such bacteria include a mutant strain which is auxotrophic for L-isoleucine and L-methionine, and is resistant to D-ribose, purine nucleoside, or pyrimidine ribonucleoside (FERM P-1841, P-5556; JP53-025034A), and a mutant strain resistant to polyketonoid (FERM P-9325; JP04-045314B).

Examples of bacteria having an ability to produce L-alanine include a Coryneform bacterium strain which is deficient in H⁺-ATPase activity (Appl Microbiol Biotechnol. November 2001; 57(4):534-40) or a Coryneform bacterium strain in which aspartic acid β-decarboxylase gene is amplified (JP07-163383A).

Furthermore, it it preferable that the bacterium of the present invention is further modified to have reduced activity of 6-phosphofructokinase. 6-phosphofructokinase activity can be measured by the method described by, for example, Denise Kotlars and Henri Buc (Methods in Enzymology (1982) 90: 60-70). The activity of 6-phosphofructokinase in the bacterium of the present invention is reduced less than that of a wild-type or non-modified strain, preferably less than 90%, more preferably less than 70%, and most preferably not less than 50% of a wild-type or non-modified strain. The activity of 6-phosphofructokinase can be reduced by mutating or disrupting a gene encoding this enzyme, wherein said method can be similar to the method used to reduce α-ketoglutarate dehydrogenase activity.

In the present invention, useful metabolites can be produced by cultivating the above-mentioned bacteria in a culture medium, allowing accumulation of the useful metabolite into the culture medium and/or bacterial cells, and collecting said useful metabolite from the culture medium and/or bacterial cells.

Cultivation, collection, and purification of useful metabolites from the medium, may be performed by conventional fermentation methods wherein a useful metabolite is produced using a bacterium. The culture medium may either be synthetic or natural, so long as the medium contains a carbon source and a nitrogen source and minerals and, if necessary, appropriate amounts of nutrients which the bacterium requires for growth. The carbon source includes various carbohydrates such as glucose and sucrose, and various organic acids. Depending on the mode of assimilation of the chosen bacterium, alcohol, including ethanol and glycerol, may be used. As the nitrogen source, ammonia, ammonium sulfate, other nitrogen compounds such as amines, a natural nitrogen source such as peptone, soybean-hydrolysate, and digested fermentative microorganism may be used. As minerals, potassium monophosphate, magnesium sulfate, sodium chloride, ferrous sulfate, manganese sulfate, calcium chloride, and the like may be used. In this invention, it is more desirable to add thiamine hydrochloride (vitamin B1) into the culture medium. The concentration of thiamine hydrochloride is more than 10 μg/L, preferably more than 10 mg/L, more preferably less than 100 mg/L.

The cultivation is performed preferably under aerobic conditions such as a shaking culture, and stirring culture with aeration, at a temperature of 20 to 42° C., preferably 37 to 40° C. The pH of the culture medium is usually between 5 and 9, preferably between 6.5 and 7.2. The pH of the culture medium can be adjusted with ammonia, calcium carbonate, various acids, various bases, and buffers. Usually, 1 to 5-day cultivation is sufficient for the accumulation of the target useful metabolite in the liquid medium.

After cultivation, solids such as cells can be removed from the liquid medium by centrifugation or membrane filtration, and then the target useful metabolite can be collected and purified by ion-exchange, concentration, and crystallization methods etc.

EXAMPLES

The present invention will be explained more specifically below with reference to the following non-limiting Examples. Abbreviations: Pyr—pyruvate, PEP—phosphoenolpyruvate, GA-3P—glyceraldehydes-3-phosphate, AceCoA—acetyl-CoA. Theoretical weight yield (Y) is calculated as Y=(molecular weight of product×moles)/(molecular weight of substrate×moles). Equations are simplified and show only carbon compounds and energy molecules.

Example 1 Calculation of Acetyl-CoA Formation by Bacterium With or Without Phosphoketolases Activities

1. Reactions of Acetyl-CoA Biosynthetic Pathway Which Does Not Involve Phosphoketolases Activities.

PTS system followed by glycolysis gives the following equation: Glucose=PEP+Pyr+ATP+2 NADH

Phosphoenolpyruvate carboxylase encoded by ppc gene catalyzes the following reaction: PEP+CO₂=oxaloacetate

Pyruvate dehydrogenase encoded by pdh gene catalyzes the following reaction: Pyr=acetyl-CoA+CO₂+NADH

And final equation is: Glucose=acetyl-CoA+oxaloacetate+ATP+3 NADH   (A)

2. Reactions of acetyl-CoA biosynthetic pathway which involves phosphoketolases activities.

PTS system followed by glycolysis gives the following equation: Glucose+PEP=fructose-6-phosphate+Pyr

Phosphoenolpyruvate synthase catalyzes the following reaction: Pyr+ATP=PEP

Phosphoenolpyruvate carboxylase coded by ppc gene catalyzes the following reaction: PEP+CO₂=oxaloacetate

Fructose-6-phosphate phosphoketolase: Phosphate+fructose-6-phosphate=H₂0+erythrose-4-phosphate+acetyl phosphate

Transaldolase and transketolase: fructose-6-phosphate+erythrose-4-phosphate=2 xylulose-5-phosphate

D-xylulose-5-phosphate phosphoketolase: Phosphate+xylulose-5-phosphate=glyceraldehyde-3-phosphate+acetyl phosphate

Glycolysis: Glyceraldehyde-3-phosphate=PEP+2 ATP+2NADH

Phosphate acetyltransferase encoded by pta gene Acetylphosphate=acetyl-CoA

And final equation is: 2 glucose+2 CO₂=3 acetyl-CoA+2 oxaloacetate+NADH   (B)

Comparison of equations (A) and (B) shows that using the activities of phosphoketolases allows generation of 1.5 molecules of acetyl-CoA from 1 molecule of glucose and saves 1 molecule of CO₂ in the contrast to the 1 molecule of acetyl-CoA obtained from 1 molecule of glucose in the glucose metabolism without the activities of phosphoketolases.

Example 2 Calculation of Theoretical Yield of L-Glutamic Acid Production Using Bacterium with or without Phosphoketolases Activities

1. Reactions of L-Glutamic Acid Biosynthetic Pathway Which Involves PTS, Glycolysis, and the TCA Cycle.

PTS+glycolysis: glucose=acetyl-CoA+oxaloacetate+ATP+3NADH   (A)

TCA cycle: acetyl-CoA+oxaloacetate=2-oxoglutarate+NADPH+CO₂

Glutamate dehydrogenase: 2-oxoglutarate+NH3+NADPH=glutamic acid

Final equation: glucose=glutamic acid+ATP+3 NADH+CO₂   (C)

Theoretical weight yield of L-glutamic acid is 81.7%

2. Reactions of L-glutamic acid biosynthetic pathway which involves glycolysis, non-oxidative PPC, and D-xylulose-5-phosphate phosphoketolase only

Glycolysis and non oxidative PPC: 5 glucose+5 PEP=5 fructose-6-phosphate+5 Pyr, where fructose-6-phosphate+ATP=2 GA-3P 2 fructose-6-phosphate+2 GA-3P=2 xylulose-5-phosphate+2 erythrose-4-phosphate 2 fructose-6-phosphate+2 erythrose-4-phosphate=4 xylulose-5-phosphate

Summary equation: 5 glucose+5 PEP+ATP=6 xylulose-5-phosphate+5 Pyr

Then, D-xylulose-5-phosphate phosphoketolase: Xylulose-5-phosphate+phosphate=GA-3P+acetylphosphate

Glycolysis and phosphate acetyltransferase: GA-3P=PEP+ATP+NADH Acetylphosphate=acetyl-CoA

Summary equation: 5 glucose=6 acetyl-CoA+PEP+5 ATP+6 NADH+5 PYR   (1) Phosphoenolpyruvate synthase PYR+2 ATP=PEP+phosphate Presuming that NADH=2 ATP

Summary equation: 5 glucose=6 acetyl-CoA+6 PEP+7 ATP

Phosphoenolpyruvate carboxylase (ppc): PEP+CO₂ =oxaloacetate

Summary equation: 5 glucose+6 CO₂=6 acetyl-CoA+6 oxaloacetate+7 ATP   (2)

TCA cycle: Oxaloacetate+acetyl-CoA=oxoglutarate+CO₂+NADPH

Summary equation: 5 glucose=6 oxoglutarate+6NADPH+7ATP

Glutamate dehydrogenase: Oxoglutarate+NADPH=glutamic acid

Final equation: 5 glucose=6 glutamic acid+7 ATP   (D) Theoretical weight yield of L-glutamic acid is 98%

3. Reactions of L-glutamic acid biosynthetic pathway which involve glycolysis, non-oxidative PPC, both phosphoketolases and glyoxalate bypass

PTS+glycolysis: 2 glucose+2 PEP=2 fructose-6-phosphate+2 PYR

fructose-6-phosphate phosphoketolase2: fructose-6-phosphate+phosphate=erythrose-4-phosphate+acetylphosphate

Transaldolase and transketolase: fructose-6-phosphate+erythrose-4-phosphate=2 xylulose-5-phosphate

Xylulose-5-phosphate phosphoketolase: 2 xylulose-5-phosphate+2 phosphate=2 GA-3P+2 acetylphosphate

Summary equation: 2glucose+2PEP=2 GA-3P+3 acetylphosphate+2 PYR

Glycolysis: GA-3P=PEP+ATP+NADH

Summary equation: 2 glucose=3 acetylphosphate+2 PYR+2 ATP+2 NADH   (3)

Phosphoenolpyruvate synthase: PYR+2 ATP=PEP

Phosphate acetyltransferase: Acetylphospate=acetyl-CoA

Summary equation: 2 glucose=3 acetyl-CoA+2 PEP+NADH

Phosphoenolpyruvate carboxylase: PEP+CO₂=oxaloacetate

Summary equation: 2 glucose+2 CO₂=3 acetyl-CoA+2 oxaloacetate+NADH or 6 glucose+6 CO₂=9 acetyl-CoA+6 oxaloacetate+3 NADH   (4)

Glyoxalate bypass: 2 Acetyl-CoA=succinate+NADH

TCA cycle: Succinate=oxaloacetate+ATP+NADH

Summary equation: 6 glucose+6 CO₂=7 acetyl-CoA+7 oxaloacetate+ATP+5 NADH

TCA cycle: Acetyl-CoA+oxaloacetate=2-oxoglutarate+NADPH+CO₂

Glutamate dehydrogenase: 2-oxoglutarate+NH₃+NADPH=glutamic acid

Final equation: 6 glucose=7 glutamic acid+ATP+5 NADH+CO₂ or 6 glucose=7 glutamic acid+11 ATP+CO₂   (E) Theoretical weight yield of L-glutamic acid is 95.3%

Comparison of equations (C), (D), and (E) shows that using the activities of phosphoketolases significantly increases theoretical yield of L-glutamic acid biosynthesis and allows generation of 1 more molecule of L-glutamic acid than molecules of glucose utilized and prevents CO₂ release in contrast to 1 molecule of L-glutamic acid obtained from 1 molecule of glucose in the glucose metabolism without the activities of phosphoketolases.

Example 3 Calculation of Theoretical Yield of Succinate Production Using a Bacterium with or without Phosphoketolases Activities

1. Reactions of Succinate Biosynthetic Pathway Which Involves PTS, Glycolysis, and the TCA Cycle.

PTS+glycolysis: glucose=PEP+PYR+ATP+2 NADH

Phosphoenolpyruvate carboxylase: PEP+CO₂=oxaloacetate

Pyruvate dehydrogenase: PYR=acetyl-CoA+CO₂+NADH

Summary equation: Glucose=oxaloacetate+acetyl-CoA+ATP+3 NADH

TCA cycle: Oxaloacetate+acetyl-CoA=succinate+NADPH+NADH+ATP+2 CO₂

Final equation: Glucose=succinate+NADPH+4 NADH+2 ATP+2 CO₂ or presuming that NADH=NADPH=2 ATP Glucose=succinate+12 ATP+2 CO₂   (F) Theoretical weight yield of succinate is 65%.

2. Reactions of succinate biosynthetic pathway which involve glycolysis, non-oxidative PPC, and D-xylulose-5-phosphate phosphoketolase only

Glycolysis, non oxidative PPC, D-xylulose-5-phosphate phosphoketolase, phosphate acetyltransferase and phosphoenolpyruvate carboxylase gives summary equation (see Example 2, equation 2): 5 glucose+6 CO₂=6 acetyl-CoA+6 oxaloacetate+7 ATP

TCA cycle: Oxaloacetate+acetyl-CoA=succinate+NADPH+NADH+ATP+2 CO₂

Final equation: 5 glucose=6 succinate+6 CO₂+6 NADPH+25 ATP or presuming that NADH=NADPH=2ATP 5 glucose=6 succinate+6 CO₂+37 ATP   (G) Theoretical weight yield of succinate is 79%

3. Reactions of succinate biosynthetic pathway which involve glycolysis, non-oxidative PPC, both phosphoketolases and glyoxalate bypass

Glycolysis, non-oxidative PPC, fructose-6-phosphatephosphoketolase, D-xylulose-5-phosphate phosphoketolase, phosphate acetyltransferase, and phosphoenolpyruvate carboxylase gives summary equation (see Example 2, equation 4): 2 glucose+2 CO₂=3 acetyl-CoA+2 oxaloacetate+NADH or 4 glucose+4 CO₂=6 acetyl-CoA+4 oxaloacetate+2 NADH

TCA cycle: Oxaloacetate+acetyl-CoA=succinate+NADPH+NADH+ATP+2 CO₂

Summary equation: 4 glucose=4 succinate+2 acetyl-CoA+4 CO₂+4NADPH+6 NADH

Glyoxylate bypass: 2 acetyl-CoA=succinate+NADH

Final equation: 4 glucose=5 succinate+4 CO₂+4 NADPH+7 NADH or presuming that NADH=NADPH=2ATP 4 glucose=5 succinate+4 CO₂+22 ATP   (H) Theoretical weight yield of succinate is Y=82%

Comparison of equations (F), (G), and (H) shows that using the activities of phosphoketolases significantly increases theoretical yield of succinate biosynthesis and allows generation of 1 more molecule of succinate than molecules of glucose utilized, and prevents CO₂ release in contrast to 1 molecule of succinate obtained from 1 molecule of glucose in the glucose metabolism without the activities of phosphoketolases.

Example 4 Calculation of Theoretical Yield of L-Leucine Production using Bacterium with or without Phosphoketolases Activities

1. Reactions of L-Leucine Biosynthetic Pathway Which Involves PTS and Glycolysis.

Glycolysis: glucose=2 PYR+2 ATP+2 NADH

L-leucine biosynthesis: 2 PYR+NADPH=2-keto-isovalerate+CO₂ 2-keto-isovalerate+acetyl-CoA+L-glutamic acid=leucine+NADH+2-oxoglutarate+CO₂ or 2-keto-isovalerate+acetyl-CoA=leucine+NADH−NADPH+CO₂ where NADPH is used for regeneration of L-glutamic acid Pyruvate dehydrogenase PYR=acetyl-CoA+NADH+CO₂

Summary equation: 3 PYR=leucine+2 NADH−2 NADPH+3 CO₂

Final equation: 3 glucose=2 leucine+4 NADH−4 NADPH+6 CO₂+6ATP+6 NADH or presuming that NADH=NADPH=2 ATP 3 glucose=2 leucine+18 ATP+6 CO₂   (1) Theoretical weight yield of L-leucine is Y=48%

2. Reactions of L-leucine biosynthetic pathway which involves glycolysis, non oxidative PPC, and D-xylulose-5-phosphate phosphoketolase only

Glycolysis, non oxidative PPC, D-xylulose-5-phosphate phosphoketolase, and phosphate acetyltransferase gives summary equation (see Example 2, equation 1): 5 glucose=6 acetyl-CoA+PEP+5 ATP+6 NADH+5 PYR

Pyruvate kinase: PEP=PYR+ATP

Summary equation: 5 glucose=6 acetyl-CoA+6PYR+6ATP+6NADH

L-leucine biosynthesis: 2 PYR+acetyl-CoA=leucine+NADH-2 NADPH+2 CO₂

Summary equation: 5 glucose=3 leucine++3 acetyl-CoA+6 ATP+9 NADH−6 NADPH+6 CO₂   (5)

Another act of glycolysis gives: 3 glucose=6PYR+18 ATP (6)

Adding equations 5 and 6: 3 glucose=6 PYR+18 ATP+5 glucose=3 leucine++3 acetyl-CoA+6 ATP+9 NADH−6 NADPH+6 CO₂=8 glucose=3 leucine+(6 PYR+3Ace-CoA)+9 NADH−6 NADPH+24 ATP+6 CO₂

Final equation: 8 glucose=6 leucine+24 ATP+12 NADH−12 NADPH+12 CO₂ or presuming that NADH=NADPH 8 glucose=6 leucine+12 CO₂+24 ATP   (J) Theoretical weight yield of L-leucine is Y=55%

3. Reactions of L-leucine biosynthetic pathway which involves glycolysis, non-oxidative PPC, and both phosphoketolases.

Glycolysis, non oxidative PPC, fructose-6-phosphatephosphoketolase, D-xylulose-5-phosphate phosphoketolase gives summary equation (see Example 2, equation 3): 2 glucose=3 acetylphosphate+2 PYR+2 ATP+2 NADH

Phosphate acetyltransferase: Acetylphospate=acetyl-CoA

Summary equation: 2 glucose=3 acetyl-CoA+2 PYR+2 ATP+2 NADH

L-leucine biosynthesis: 2 PYR+acetyl-CoA=leucine+NADH−2 NADPH+2 CO₂

Summary equation: 2 glucose=leucine+2 acetyl-CoA+2 ATP+3 NADH+−2 NADPH+2 CO₂   (7)

Another act of glycolysis gives: 2 glucose=3 PYR+12 ATP   (8)

Adding equations 7 and 8: 2 glucose=leucine+2 acetyl-CoA+2 ATP+3 NADH+−2 NADPH+2 CO₂+2 glucose=4 PYR+12 ATP=4 glucose=leucine+(4 PYR+2 acetyl-CoA)+14 ATP+3 NADH+−2 NADPH+2 CO₂

L-leucine biosynthesis: 4 PYR+2 acetyl-CoA=2 leucine+2 NADH−4 NADPH+4 CO₂

Final equation: 4 glucose=3 leucine+14 ATP+5 NADH+−6 NADPH+6 CO₂ or presuming that NADH=NADPH=2 ATP 4 glucose=3 leucine+6 CO₂+12 ATP   (K) Theoretical weight yield of L-leucine is Y=55%

Comparison of equations (I), (J), and (K) shows that using the activities of phosphoketolases significantly increases theoretical yield of L-leucine biosynthesis.

Example 4 Cloning of Phosphoketolase Gene (xpk1) from Lactobacillus plantarum and Evaluation of the Effect of xpk1 Gene Amplification on Production of Useful Metabolites by E. coli

The xpk1 gene can be cloned from chromosomal DNA of the Lactobacillus plantarum strain 8PA3 (VKPM B-7495). Based on the reported nucleotide sequence, the primers depicted in SEQ ID No. 7 (primer 1) and No. 8 (primer 2) for amplification of xpk1 gene can be synthesized. The primer 1 contains a HindIII recognition site introduced at the 5′-end thereof. The primer 2 contains EcoRI recognition site introduced at the 5′-end thereof.

The chromosomal DNA of Lactobacillus plantarum strain 8PA3 can be used as a template for PCR and can be prepared by an ordinary method. PCR can be carried out using “Applied Biosystems GeneAmp PCR System 2400” under the following conditions: initial DNA denaturation at 95° C. for 5 min; then 30 cycles of denaturation at 95° C. for 30 sec, annealing at 55° C. for 60 sec and elongation at 72° C. for 120 sec; the final polymerization for 7 min at 72° C. using Fermentas Taq polymerase (Fermentas, Lithuania). The obtained PCR fragment containing xpk1 gene with its own SD sequence and without a promoter sequence can be treated with HindIII and EcoRI and inserted in the vector pMW119 previously treated with the same enzymes. Thus, the plasmid pMW-xpk1 can be obtained.

Transformation of an useful metabolite-producing bacterial strain with the pMW-xpk1 plasmid can be performed by an ordinary method to obtain the strain containing amplified xpk1 gene.

Production of L-glutamic acid by E. coli strain VL334thrC⁺-pMW-xpk1.

Transformation of the E. coli L-glutamic acid producing strain VL334thrC⁺ (European patent publication No. 1172433) with the pMW-xpk1 plasmid can be performed by an ordinary method to obtain the strain VL334thrC⁺-pMW-xpk1.

Both strains, VL334thrC⁺ and VL334thrC⁺-pMW-xpk1, can be grown for 18-24 hours at 37° C. on L-agar plates. Then, one loop of the cells can be transferred into test tubes containing 2 ml of fermentation medium. The fermentation medium should contain 60 g/l glucose, 25 g/l ammonium sulfate, 2 g/l KH₂PO₄, 1 g/l MgSO₄, 0.1 mg/ml thiamine, 70 μg/ml L-isoleucine and 25 g/l chalk (pH 7.2). Glucose and chalk should be sterilized separately. Cultivation can be carried out at 30° C. for 3 days with shaking. After cultivation, the amount of L-glutamic acid which has accumulated can be determined by paper chromatography (liquid phase composition: butanol-acetic acid-water=4:1:1) with subsequent staining by ninhydrin (1% solution in acetone) and further elution of compounds in 50% ethanol with 0.5% CdCl₂.

Production of L-proline by E. coli strain 702ilvA-pMW-xpk1.

Transformation of the E. coli L-proline producing strain 702ilvA (VKPM B-8012, Russian patent application 2000124295, European patent publication NO. 1172433) with the pMW-xpk1 plasmid can be performed by an ordinary method to obtain the strain 702ilvA-pMW-xpk1.

Both E. coli strains 702ilvA and 702ilvA-pMW-xpk1 can be grown for 18-24 hours at 37° C. on L-agar plates. Then, these strains can be cultivated in the same conditions as described above.

Production of L-leucine by E. coli strain 57-pMW-xpk1.

Transformation of the E. coli L-leucine producing strain 57 (VKPM B-7386, U.S. Pat. No. 6,124,121) with the pMW-xpk1 plasmid can be performed by an ordinary method to obtain the strain 57-pMW-xpk1.

Both E. coli strains 57 and 57-pMW-xpk1 can be grown for 18-24 hours at 37° C. on L-agar plates. Then, these strains can be cultivated under the same conditions as described above without addition of isoleucine in the medium.

Production of L-cysteine by E. coli strain JM15(ydeD)-pMW-xpk1.

Transformation of the E. coli L-cysteine producing strain JM15(ydeD) with the pMW-xpk1 plasmid can be performed by an ordinary method to obtain the strain JM15(ydeD)-pMW-xpk1.

E. coli strain JM15(ydeD) is a derivative of E. coli strain JM 15 (U.S. Pat. No. 6,218,168) which can be transformed with the DNA having ydeD gene coding for a membrane protein, which is not involved in biosynthetic pathway of any L-amino acid (U.S. Pat. No. 5,972,663).

Fermentation conditions for evaluation of L-cysteine production are described in detail in Example 6 of U.S. Pat. No. 6,218,168.

Production of L-glutamic acid by Pantoea ananatis strain AJ13356-xpk1.

Transformation of the L-glutamic acid-producing Enterobacter agglomerans AJ13356 strain (U.S. Pat. No. 6,331,419) (recently re-classified as Pantoea ananatis) with the pMW-xpk1 plasmid can be performed by an ordinary method to obtain the strain AJ133556-pMW-xpk1.

Each of the AJ13356 and AJ13356-xpk1 strains can be inoculated into a 500 ml-volume flask which contains 20 ml of a culture medium comprising 40 g/L glucose, 20 g/L ammonium sulfate, 0.5 g/L magnesium sulfate heptahydrate, 2 g/L potassium dihydrogenphosphate, 0.5 g/L sodium chloride, 0.25 g/L calcium chloride heptahydrate, 0.02 g/L ferrous sulfate heptahydrate, 0.02 g/L manganese sulfate tetrahydrate, 0.72 mg/L zinc sulfate dihydrate, 0.64 mg/L copper sulfate pentahydrate, 0.72 mg/L cobalt chloride hexahydrate, 0.4 mg/L boric acid, 1.2 mg/L sodium molybdate dihydrate, 2 g/L yeast extract, 30 g/L calcium carbonate, 200 mg/L L-lysine monohydrochloride, 200 mg/L L-methionine and 200 mg/L DL-α,ε-diaminopimelic acid (DAP), and can be cultured at 37° C. with shaking until the glucose contained in the culture medium is completely consumed. After the cultivation is completed, L-glutamic acid which has accumulated in the culture medium can be measured as above.

Example 5 Confirmation of Physiological Activity of Phosphoketolase Gene using Coryneform Bacterium

(5-1) Cloning of Phosphoketolase Gene and Construction of Expression Plasmid

(1) Construction of pVK9-xfp

The xfp gene was cloned from chromosomal DNA of the strain Bifidobacterium animalis JCM 1190. Based on the reported nucleotide sequence of the xfp gene of Bifidobacterium animalis ATCC 27674 (GenBank Accession No. AY518213, SEQ ID No: 9), the primers depicted in SEQ ID No: 13 and No: 14 can be synthesized and used for amplification of the xfp gene. Bifidobacterium animalis JCM1190 can be obtained from the Japan Collection of Microorganisms (JCM).

The DNA sequence of the xfp gene from B.animalis JCM1190 is shown in SEQ ID No.11.

In order to amplify the xfp gene (SEQ ID NO: 11) from Bifidobacterium animalis JCM1190 and clone it into a pVK9 E. coli -coryneform shuttle vector, chromosomal DNA of Bifidobacterium animalis JCM1190 was extracted using the Wizard Genomic Purification Kit (Promega). The pVK9 is an E. coli-coryneform shuttle vector, obtained by digesting pHK4 (JP 5-007491A) with BamHI and KpnI to obtain the region including its replicable origin and introducing the region into the AvaII site of pHSG299 (product of Takara Bio Inc.) The xfp gene fragment including a promoter region was amplified by PCR using the chromosomal DNA of B. animalis as a template and using the primers shown in SEQ ID NO: 13 and NO: 14 as primers. PCR was carried out by a conventional method.

The resulting PCR product was purified in a conventional manner and digested with Xba I. The digested PCR product was ligated by using Ligation Kit (product of Takara Bio Inc) to pVK9 which had been digested with Xba I. The ligation mixture was used to transform competent cells of Escherichia coli DH5α (product of Takara Bio Inc.). The cells were plated on LB medium (10 g of bacto-tryptone, 5 g of bacto-yeast extract and 10 g of NaCl in 1 L) containing 100 μM of IPTG, 40 mg/ml of X-Gal and 25 mg /ml of Km and cultured overnight. Then, the white colonies which emerged were selected and separated into single colonies to obtain transformants. The target plasmid pVK9-xfp containing the xfp gene was isolated from the transformants.

(2) Construction of pVK9-PS2_xfp

A DNA fragment, wherein a native promoter region of B. animalis xfp gene was replaced with a PS2 promoter (Peyret J L, Mol Microbiol. July 1993; 9(1):97-109,WO93/03158), was obtained in accordance with the overlap PCR method (R. M. Horton, H. D. Hunts, S. N. Ho, J. K. Pullen, and L. R. Pease, Gene, 77, 61-68(1989)). The method is specifically described below.

First, PCR was performed by using pPSTG1 containing the PS2 promoter (Y. Kikuchi, M. Date, K. Yokoyama, Y. Umezawa and H. Matsui, Appl. Environ. Microbiol. Appl. Environ. Microbiol 69, 358-366(2003)) as a template and the synthetic DNAs of SEQ ID NO: 15 and No: 16 as primers to obtain an amplification product of the PS2 promoter. Then, in order to obtain an amplification product of a sequence of the xfp gene (coding region), PCR was performed using the pVK9-xfp as a template and the synthetic DNAs of SEQ ID NO: 17 and NO: 18 as primers. SEQ ID NO: 16 and NO: 18 are complementary to each other.

Then, in order to obtain a fragment containing the PS2 promoter and the B. animalis xfp gene, the aforementioned gene fragments of PS2 promoter and the B. animalis xfp gene were mixed in substantially equimolar amounts, and PCR was performed using this mixture as a template and the synthetic DNAs of SEQ ID NO: 14 and NO: 19 as primers.

The resulting PCR product was purified in a conventional manner and digested with Xba I. The digested PCR product was ligated by using Ligation Kit (product of Takara Bio Inc) to pVK9 which had been digested with Xba I. The ligation mixture was used to transform competent cells of Escherichia coli DH5α (product of Takara Bio Inc.). The cells were plated on LB medium (10 g of bacto-tryptone, 5 g of bacto-yeast extract and 10 g of NaCl in 1 L) containing 100 μM of IPTG, 40 mg/ml of X-Gal and 25 mg /ml of Km and cultured overnight. Then, the white colonies which emerged were selected and separated into single colonies to obtain transformants. The object plasmid pVK9-PS2_xfp containing the PS2 promoter and xfp gene was isolated from the transformants.

(3) Construction of pVK9_tac xfp

A DNA fragment wherein the native promoter region of the B. animalis xfp gene was replaced with a tac promoter was obtained in accordance with the overlap PCR method (R. M. Horton, H. D. Hunt, S. N. Ho, J. K. Pullen and L. R. Pease, Gene, 77, 61-68(1989)). The method is specifically described below.

First, PCR was performed by using the pKK223-3 (Pharmacia) as a template and the synthetic DNAs of SEQ ID NO: 20 and NO: 21 as primers to obtain an amplification product of the tac promoter. Then, in order to obtain an amplification product of a sequence of the xfp gene (coding region), PCR was performed using the chromosomal DNA of B. animalis JCM1190 as a template and synthetic DNAs SEQ ID NO:14 and NO: 22 as primers. The nucleotide sequences of SEQ ID NO: 20 and NO: 22 are complementary to each other.

Then, in order to obtain a fragment containing the tac promoter and the B. animalis xfp gene, the aforementioned gene fragments of the tac promoter and the B. animalis xfp gene were mixed in substantially equimolar amounts, and PCR was performed using this mixture as a template and synthetic DNAs SEQ ID NO: 14 and NO: 21 as primers.

The resulting PCR product was purified in a conventional manner and digested with Xba I. The digested PCR product was ligated by using Ligation Kit (product of Takara Bio Inc) to pVK9 which had been digested with Xba I. The ligation mixture was used to transform competent cells (product of Takara Bio Inc.) of Escherichia coli DH5α. The cells were plated on LB medium (10 g of bacto-tryptone, 5 g of bacto-yeast extract and 10 g of NaCl in 1 L) containing 100 μM of IPTG, 40 mg/ml of X-Gal and 25 mg /ml of Km and cultured overnight. Then, the white colonies which emerged were selected and separated into single colonies to obtain transformants. The object plasmid pVK9-tac_xfp containing the xfp gene and tac promoter was isolated from the transformants.

(4) Construction of pVK9-PS2_xpkA

The xpkA gene was cloned from chromosomal DNA of the strain Lactobacillus pentosus JCM1558. Lactobacillus pentosus JCM1558 can be obtained from the Japan Collection of Microorganisms (JCM). In order to amplify an xpkA gene encoding phosphoketolase (SEQ ID NO: 1; AJ309011: gi: 16605513) from Lactobacillus pentosus JCM1558 and clone it into a pVK9 shuttle vector, the chromosomal DNA of Lactobacillus pentosus JCM1558 was extracted using a Wizard Genomic Purification Kit (Promega).

A DNA fragment was obtained by replacing the promoter region of the L. pentosus xpkA gene encoding phosphoketolase with a PS2 promoter in accordance with the overlap PCR method. The method is specifically described herein.

First PCR was performed by using pPSTGI containing the PS2 promoter (Y. Kikuchi, M. Date, K. Yokoyama, Y. Umezawa and H. Matsui, Appl. Environ. Microbiol. Appl. Environ. Microbiol 69, 358-366(2003)) as a template and the synthetic DNAs of SEQ ID NO: 15 and No: 23 as primers to obtain an amplification product of the PS2 promoter. Then, in order to obtain an amplification product of a sequence of the xpkA gene (coding region), PCR was performed using the chromosomal DNA of Lactobacillus pentosus JCM1558 as a template and the synthetic DNAs of SEQ ID NO: 24 and NO: 25 as primers. The nucleotide sequences of SEQ-ID NO: 23 and NO: 25 are complementary to each other.

Then, in order to obtain a fragment containing the PS2 promoter and L. pentosus xpkA gene, the aforementioned gene fragments of PS2 promoter and the L. pentosus xpkA gene were mixed in substantially equimolar amounts, and PCR was performed using this mixture as a template and the synthetic DNAs of SEQ ID NO: 19 and NO: 24 as primers.

The resulting PCR product was purified in a conventional manner and digested with Xba I. The digested PCR product was ligated by using Ligation Kit (product of Takara Bio Inc) to pVK9 which had been digested with Xba I. The ligation mixture was used to transform competent cells (product of Takara Bio Inc.) of Escherichia coli DH5α. The cells were plated on LB medium (10 g of bacto-tryptone, 5 g of bacto-yeast extract and 10 g of NaCl in 1 L) containing 100 μM of IPTG, 40 mg/ml of X-Gal and 25 mg /ml of Km and cultured overnight. Then, the white colonies which emerged were selected and separated into single colonies to obtain transformants. The object plasmid pVK9-PS2_xpkA containing the PS2 promoter and xpkA was isolated from the transformants.

(5-2)Confirmation of the in vivo Physiological Activity of Phosphoketolase

(1) Construction of ATCC13869ΔaceE strain

A PDH-defective strain is auxotrophic for acetate. On the other hand, a PDH (pyruvate dehydrogenase)-defective strain into which the phosphoketolase gene is introduced will not be auxotrophic for acetate any more. Accordingly, the effect of the introduction of the xfp or xpkA gene was confirmed.

Construction of a gene-disruption vector

(A) Construction of pBS3

With chromosomal DNA of Bacillus subtilis as a template for PCR and synthetic DNAs of SEQ ID NO: 42 and NO: 43 as primers, a sacB gene (SEQ ID NO: 40) was obtained by PCR. The PCR reaction was conducted by, after 1 cycle maintaining 94° C. for 5 minutes, repeating 25 times a cycle of 94° C. for 30 seconds, 49° C. for 30 seconds and 72° C. for 2 minutes, using LA taq (TaKaRa Bio). The PCR product was digested with Bg1II and BamHI and blunt-ended after purification by an ordinary method. The resulting PCR product was ligated with pHSG299 which had been digested with AvaII and blunt-ended. The plasmid was used to transform competent cells of Escherichia coli JM109 (Takara Bio Inc.), and then the transformants were suspended in LB medium containing 25 mg/ml of kanamycin and cultivated overnight. The colonies were picked and subjected to single colony isolation, and then transformants were obtained. The target plasmid pBS3 containing the sacB gene was isolated from the transformants.

The construction procedure of pBS3 is illustrated in FIG. 2.

(B) Construction of pBS4S

By overlap PCR, a plasmid was obtained by disrupting the SmaI site in the coding region of the kanamycin-resistant gene in the plasmid pBS3. First, using pBS3 as a template and synthetic DNAs of SEQ ID NO: 44 and NO: 45 as primers, PCR was performed, and the PCR product containing the N-terminal region of the kanamycin resistant gene was obtained. On the other hand, in order to obtain a PCR product containing the C-terminal region of kanamycin-resistance gene, PCR was performed using pBS3 as a template and synthetic DNAs of SEQ ID NO: 46 and NO: 47 as primers. The PCR product can be obtained by, after 1 cycle of heat treatment at 98° C. for 5 minutes, repeating 25 times a cycle of 98° C. for 10 seconds, 57° C. for 30 seconds, and 72° C. for 1 minute, using Pyrobest DNA Polymerase (Takara Bio Inc.). The nucleotide sequences of SEQ ID NO: 45 and NO: 46 are partially complementary to each other.

The SmaI site of this sequence was disrupted by introducing a mutation without causing an amino acid substitution. In order to obtain a mutant kanamycin-resistance gene fragment in which the SmaI site is disrupted, the N-terminal region of the gene product and the C-terminal region of the gene product of the kanamycin-resistance gene were mixed to form a nearly equimolar mixture. Using the resulting mixture as a template for PCR and synthetic DNA of SEQ ID NO: 44 and NO: 47 as primers, PCR was performed. The PCR product having the mutation in kanamycin-resistance gene was obtained. The target PCR product can be obtained by, after 1 cycle of heat treatment at 98° C. for 5 minutes, repeating 25 times a cycle comprising of 98° C. for 10 seconds, 57° C. for 30 seconds a 72° C. for 1.5 minute using Pyrobest DNA Polymerase (Takara Bio Inc.).

The PCR product was digested with BanII after purification and inserted into the BanII site of the above-described pBS3. The obtained DNA was used to transform competent cells of Escherichia coli JM109 (Takara Bio Inc.), suspended to LB medium containing 25 mg/ml of kanamycin and cultivated overnight. The colonies were picked and subjected to single colony isolation, and transformants were obtained. The object plasmid pBS4S was isolated from the transformants. The construction procedure of pBS4S is illustrated in FIG. 3.

(C) Construction of pBS5T

A plasmid pBS5T having a temperature-sensitive replication origin from coryneform bacterium was constructed by the following procedure.

The region of temperature-sensitive replication was obtained by digesting pHSC4 (U.S. Pat. No. 5,616,480A) with BamHI and SmaI and blunt-ending the digested fragment, and then this region was ligated to the blunt-ended NdeI site of pBS4S. The obtained DNA was used to transform competent cells of Escherichia coli JM109 (product of Takara Bio Inc.), and transformants were suspended in LB medium containing 25 mg/ml of Km and cultivated overnight. The colonies were picked and subjected to single colony isolation, and then transformants were obtained. The object plasmid pBS5T containing the sacB fragment and temperature-sensitive replication origin was isolated from the transformants. FIG. 4 illustrates the construction scheme of pBS5T.

(2) Cloning of a Fragment for aceE Gene-Disruption

A DNA fragment for aceE endoding pyruvate dehydrogenase (pyruvate dehydrogenase E1 component) gene disruption was obtained from ATCC13869 by the overlap PCR method using as primers synthetic DNAs based on the reported nucleotide sequence of aceE gene of Corynebacterium glutamicum ATCC13032 (GenBank Database Accession No. NC 003450; SEQ ID NO: 38).

First, PCR was performed by using a chromosomal DNA of C. glutamicum ATCC13869 as a template and the synthetic DNAs of SEQ ID NO: 26 and No: 27 as primers to obtain an amplification product of the N-terminus side of the aceE gene. Then, in order to obtain an amplification product of the C-terminus side of the aceE gene, PCR was performed using the chromosomal DNA of C. glutamicum ATCC13869 as a template and the synthetic DNAs of SEQ ID NO: 28 and NO: 29 as primers. The sequences of SEQ ID NO: 26 and NO: 28 are complementary to each other.

Then, in order to obtain an aceE fragment in which an internal sequence is deleted, the aforementioned gene fragments of the N- and C-terminus were mixed in substantially equimolar amounts, and PCR was performed using this mixture as a template and the synthetic DNAs of SEQ ID NO: 30 and NO: 31 as primers.

The resulting PCR product was purified in a conventional manner and digested with Sma I. The digested PCR product was ligated by using Ligation Kit (product of Takara Bio Inc) to the above-described pBS5T which had been digested with Sma I. The ligation mixture was used to transform competent cells of Escherichia coli DH5α (product of Takara Bio Inc.). The cells were plated on LB medium (10 g of bacto-tryptone, 5 g of bacto-yeast extract and 10 g of NaCl in 1 L) containing 100 μM of IPTG, 40 mg/ml of X-Gal and 25 mg/ml of Km and cultured overnight. Then, the white colonies which emerged were selected and separated into single colonies to obtain transformants. The object plasmid pBS5T-ΔaceE was isolated from the transformants.

(3) Construction of aceE-Disrupted Strain

aceE gene encodes pyruvate dehydrogenase E1 component.

First, the ATCC13869 strain was transformed with a high concentration of plasmid pBS5T-ΔaceE by the electric pulse method, plated on the CM-Dex medium (5 g/L of glucose, 10 g/L of polypeptone, 10 g/L of yeast extract, 1 g/L of KH₂PO₄, 0.4 g/L of MgSO₄.7H₂O, 0.01 g/L of FeSO₄.7H₂O, 0.01 g/L of MnSO₄.7H₂O, 3 g/L of urea, 1.2 g/L of soybean hydrolysate and 10 mg/L of biotin, pH 7.5 (NaOH)) containing 25 mg/ml of kanamycin, and cultured at 25° C. for about 60 hours, and colonies which emerged were isolated as transformants. Next, the transformants were cultured on a CM-Dex liquid medium at 34° C. overnight. After appropriate dilution, the cultured transformant was suspended to a CM-Dex medium containing 25 mg/ml of kanamycin and cultured at 34° C. for about 30 hours. The ΔaceE strain, which can grow in this medium and has both the kanamycin-resistant gene and the sacB gene derived from the plasmid on the chromosome, was obtained as a result of single cross-over homologous recombination between the disrupted-type of aceE gene (ΔaceE) on the plasmid and the native aceE gene on the chromosome.

Next, the ΔaceE strain obtained by single cross-over homologous recombination was cultured overnight on a CM-Dex liquid medium at 31.5° C. After appropriate dilution, the single cross-over homologous recombinant ΔaceE strain was suspended in a kanamycin-free and 10%-sucrose-containing Dex-S 10 medium (10 g/L of sucrose, 10 g/L of polypeptone, 10 g/L of yeast extract, 1 g/L of KH₂PO₄, 0.4 g/L of MgSO₄.7H₂O, 0.01 g/L of FeSO₄.7H₂O, 0.01 g/L of MnSO₄.4H₂O, 3 g/L of urea, 1.2 g/L of soybean hydrolysate, 10 mg/L of biotin and 2 g/l of sodium acetate, adjusted to pH 7.5 by KOH). Culture was performed at 34° C. for about 30 hours. As a result of second cross-over homologous recombination, a strain which does not have the chromosomal SacB gene and is not sensitive to sucrose was obtained.

The strains obtained in this manner include those having the disrupted-type of aceE gene and those having the wild-type aceE gene. Whether the aceE gene is the disrupted-type or the wild-type was confirmed by a direct PCR reaction using the cells obtained by culturing on a Dex-S10 agar medium. The strain containing the disrupted-type of aceE gene was selected and named “ATCC13869ΔaceE”.

(4) Evaluation of Physiological Activity of Phosphoketolase in ΔaceE Strain

pVK9 (plasmid for control), pVK9-xfp (plasmid for amplifying xfp gene of Bifidobacterium animalis), pVK9-PS2_xfp (plasmid for amplifying xfp gene of Bifidobacterium animalis, in which its native promoter is replaced with a PS2 promoter), and pVK9-PS2_xpkA (plasmid for amplifying xpkA gene of Lactobacillus pentosus, in which its native promoter is replaced with a PS2 promoter) were each introduced into the ATCC13869ΔaceE strain to obtain phosphoketolase-expressing strains, and pVK9 was introduced into the ATCC13869 strain as the control. Specifically, the ATCC13869ΔaceE strain and the ATCC13869 strain were each transformed by the electric pulse method, plated on a CM-Dex medium (5 g/L of glucose; 10 g/L of polypeptone, 10 g/L of yeast extract, 1 g/L of KH₂PO₄, 0.4 g/L of MgSO₄.7H₂O, 0.01 g/L of FeSO₄.7H₂O, 0.01 g/L of MnSO₄.7H₂O, 3 g/L of urea, 1.2 g/L of soybean hydrolysate and 10 mg/L of biotin, adjusted to pH 7.5 by NaOH) containing 25 mg/ml of kanamycin, and cultured at 31.5° C. for about 30 hours. The colonies which emerged were isolated as transformants and designated ATCC13869ΔaceE(pVK9), ATCC13869ΔaceE(pVK9-xfp), ATCC13869ΔaceE(pVK9-PS2_xfp), ATCC13869ΔaceE(pVK9-PS2_xpkA) and ATCC1869(pVK9), respectively.

These strains were cultured in a micro-shaker (“Biophotorecorder TN-1506”, product of ADVANTEC) while measuring OD with the function of time. Two mediums are employed here, that is, a minimum liquid medium (composed of 10 g/L of glucose, 2.5 g/L of (NH₄)₂SO₄, 0.5 g/L of KH₂PO₄, 0.25 g/L of MgSO₄.7H₂O, 0.01 g/L of FeSO₄.7H₂O, 0.01 g/L of MnSO₄.7H₂O, 2 g/L of urea, 50 mg/L of Biotin, 100 mL of VB1-HCl, 15 mg/L of protocatechuic acid, 0.02 mg/L of CuSO₄, 10 mg/L of CaCl₂, and 40 g/L of MOPS, adjusted to pH 7.0 by KOH) containing 25 mg/ml of kanamycin, and a minimum liquid medium containing acetate (2 g/L of sodium acetate). The results of the cultivation on the minimum medium and the results of the cultivation on the minimum medium containing acetate are illustrated in FIGS. 6 and 5, respectively. The ATCC13869ΔaceE(pVK9) strain could grow on the minimum medium containing acetate, but could not grow on the minimum medium, and thus exhibited acetate-auxotrophy. The ATCC13869ΔaceE strains carrying pvK9-xfp, pVK9-PS2_xfp, and pVK9-PS2_xpkA, respectively could grow even on the minimum medium, indicating that introduction of xfp or xpkA results in compensation for the acetate-auxotrophy of the PDH-defective strain. Accordingly, the physiological activity of phosphoketolase which had been introduced into C. glutamicum was confirmed.

Example 6 Confirmation of Enzyme Activity of Phosphoketolase

The Coryneform bacterium can produce L-glutamic acid under conditions where biotin is limited, or penicillin or surfactant is added in the medium (refer to WO95/34672). It is known, on the other hand, that the strain which was modified to decrease α-ketoglutarate dehydrogenase activity (E1.2.4.2 sucA; 2-oxoglutarate dehydrogenase) can produce L-glutamic acid even not under such conditions (Kimura E., Adv. Biochem. Eng. Biotechnol., 79, 37-57(2003), “Metabolic engineering of glutamic acid production”). Accordingly, we studied the improvement in the fermentation yield of L-glutamic acid by the introduction of a phosphoketolase gene using a sucA-deficient strain C.glutamicum ATCC13869.

(6-1) Construction of the ATCC13869ΔsucA Strain

(1) Cloning of a Fragment for Disruption of sucA Gene

A sucA-deleted fragment derived from the strain C. glutamicum ATCC13869 was obtained by the overlap PCR method using as primers synthetic DNAs designed based on the nucleotide sequence sucA gene of C. glutamicum ATCC13032 (GenBank Database Accession No. NC_(—)003450; SEQ ID NO: 48).

First, PCR was performed by using a chromosomal DNA of C. glutamicum ATCC13869 as a template and the synthetic DNAs of SEQ ID NO: 32 and No: 33 as primers to obtain an amplification product of the N-terminus of the sucA gene. Then, in order to obtain an amplification product of the C-terminus of the sucA gene, PCR was performed using the chromosomal DNA of C. glutamicum ATCC13869 as a template and the synthetic DNAs of SEQ ID NO: 34 and NO: 35 as primers. The sequences of SEQ ID NO: 33 and NO: 34 are complementary to each other.

Then, in order to obtain a sucA fragment in which its internal sequence is deleted, the aforementioned gene fragments of the N- and C-terminus were mixed in substantially equimolar amounts, and PCR was performed using this mixture as a template and the synthetic DNAs of SEQ ID NO: 36 and NO: 37 as primers.

The resulting PCR product was purified in a conventional manner and digested with BamHI. The digested PCR product was ligated by using Ligation Kit (product of Takara Bio Inc) to the above-described pBS3 which had been digested with BamHI. The ligation mixture was used to transform competent cells of Escherichia coli DH5α (product of Takara Bio Inc.). The cells were plated on LB medium (10 g of bacto-tryptone, 5 g of bacto-yeast extract and 10 g of NaCl in 1 L) containing 100 μM of IPTG, 40 mg/ml of X-Gal and 25 mg/ml of Km and cultured overnight. Then, the white colonies which emerged were selected and separated into single colonies to obtain transformants. The target plasmid pBS3ΔsucA was isolated from the transformants.

(2) Preparation of a sucA-Disrupted Strain

The pBS3ΔsucA prepared in (A) does not contain a region autonomously replicable in the cells of coryneform bacterium. Therefore, if a corynebacterium is transformed with this plasmid, a strain having this plasmid integrated into its chromosome by homologous recombination would appear as a transformant although it occurs at an extremely low frequency. C. glutamicum ATCC13869 was transformed with a high concentration of the plasmid pBS3ΔsucA by the electric pulse method, plated on CM-Dex medium (5 g/L of glucose, 10 g/L of polypeptone, 10 g/L of yeast extract, 1 g/L of KH₂PO₄, 0.4 g/L of MgSO₄.7H₂O, 0.01 g/L of FeSO₄.7H₂O, 0.01 g/L of MnSO₄.7H₂O, 3 g/L of urea, 1.2 g/L of soybean hydrolysate and 10 mg/L of biotin, adjusted to pH 7.5 by NaOH) containing 25 mg/ml of kanamycin, and cultured at 31.5° C. for about 30 hours, and then, the colonies which emerged were isolated as transformants. These transformants have both the kanamycin-resistance gene and the SacB gene derived from the plasmid, as a result of homologous recombination between the sucA gene fragment of the plasmid and the native gene on the chromosome.

Next, the obtained first recombinant strain was cultured overnight on a kanamycin-free CM-Dex liquid medium at 31.5° C. After appropriate dilution, the cultured transformant was plated on the kanamycin-free Dex-S10 medium (10 g/L of sucrose, 10 g/L of polypeptone, 10 g/L of yeast extract, 1 g/L of KH₂PO₄, 0.4 g/L of MgSO₄.7H₂O, 0.01 g/L of FeSO₄.7H₂O, 0.01 g/L of MnSO₄.4H₂O, 3 g/L of urea, 1.2 g/L of soybean hydrolysate, and 10 mg/L of biotin, adjusted to pH 7.5 by KOH) containing 10% sucrose, cultured at 31.5° C. for about 30 hours, and the colony which emerged was isolated as a transformant. As a result of the second cross-over homologous recombination, a strain which has lost the SacB gene from the chromosome and is not sensitive to sucrose was obtained.

The strains obtained in this manner include those having a disrupted-type of sucA gene and those having a wild-type sucA gene. Whether the sucA gene is the disrupted-type or the wild-type was confirmed by a direct PCR reaction using the cells obtained by culturing on a Dex-S10 agar medium. The strain having a disrupted-type of sucA gene was selected.

The production of L-glutamic acid by the sucA-disrupted strain was evaluated by the following method. The cells of the sucA-disrupted strain obtained by culturing on a CM2B plate medium were inoculated into a flask containing 80 g of glucose, 1 g of KH₂PO₄, 0.4 g of MgSO₄, 30 g of (NH₄)₂SO₄, 0.01 g of FeSO₄.7H₂O, 0.01 g of MnSO₄.7H₂O, 15 ml of soybean hydrolysate solution, 200 mg of thiamine hydrochloride, 60 mg of biotin and 50 g of CaCO₃ in 1L of pure water (adjusted to pH 8.0 with KOH) and cultured at 31.5° C. with shaking until the suger was completely consumed. After completion of the culture, the amount of accumulated L-glutamic acid in the culture broth was measured. The sucA-disrupted strain which can produce a greater fermentation yield of L-glutamic acid than ATCC13869 was selected as the sucA-disrupted strain of ATCC13869 and named ATCC13869ΔsucA.

(6-2) Confirmation of Phosphoketolase Gene Expression and Enzyme Activity in sucA-Disrupted ATCC13869 Strain

Expression of a phosphoketolase gene and its enzyme activity was investigated by using the strain which had been modified to enhance the expression of phosphoketolase.

pVK9 (plasmid for control), pVK9-xfp (plasmid for amplifying xfp gene of Bifidobacterium animalis), pVK9-tac_xfp (plasmid for amplifying xfp gene of Bifidobacterium animalis, in which its native promoter is replaced with a tac promoter), pVK9-PS2_xfp (plasmid for amplifying xfp gene of Bifidobacterium animalis, in which its native promoter is replaced with a PS2 promoter), and pVK9-PS2_xpkA (plasmid for amplifying xpkA gene of Lactobacillus pentosus, in which its native promoter is replaced with a PS2 promoter) were each introduced into the above-described C. glutamicum ATCC13869ΔsucA to obtain phosphoketolase-expressing strains. Specifically, the ATCC13869ΔsucA strain was transformed with the each of the plasmids by the electric pulse method, plated on a CM-Dex medium (5 g/L of glucose, 10 g/L of polypeptone, 10 g/L of yeast extract, 1 g/L of KH₂PO₄, 0.4 g/L of MgSO₄.7H₂O, 0.01 g/L of FeSO₄.7H₂O, 0.01 g/L of MnSO₄.7H₂O, 3 g/L of urea, 1.2 g/L of soybean hydrolysate and 10 mg/L of biotin, adjusted to pH 7.5 by NaOH) containing 25 mg/ml of kanamycin and cultured at 31.5° C. for about 30 hours. The colonies which emerged were isolated as transformants and designated as ATCC13869ΔsucA(pVK9), ATCC13869ΔsucA(pVK9-xfp), ATCC13869ΔsucA(pVK9-tac_xfp), ATCC13869ΔsucA(pVK9-PS2_xfp), and ATCC13869ΔsucA(pVK9-PS2_xpkA), respectively.

In order to obtain a crude enzyme solution, the cells of the aforementioned strains obtained by culturing each strain on a CM-Dex plate medium were inoculated into a Flask containing 30 g of glucose, 1 g of KH₂PO₄, 0.4 g of MgSO₄, 15 g of (NH₄)₂SO₄, 0.01 g of FeSO₄.7H₂O, 0.01 g of MnSO₄.7H₂O, 13.7 ml of a soybean hydrolysate solution, 200 mg thiamine hydrochloride, 300 mg of biotin, and 50 g of CaCO₃ in 1 L of pure water (adjusted to pH 8.0 with KOH) and cultured at 31.5° C. with shaking. The cultivation was terminated when the OD620 of cell concentration became 15, as measured by “Hitachi Spectrophotometer U-2000A”. After removal of calcium carbonate by moderate centrifugation at 3000 rpm for 30 seconds, the cells were collected. The following procedures were carried out at 0 to 4° C. The collected cells were washed twice with a 0.85N NaCl solution and resuspended in 4 ml/g (wet weight) of buffer A (100 mM KPO₄ (pH 6.5), 30 mM KCl, 0.1 mM EDTA, 1 mM MgCl2, 0.2 mM PMSF, 2 mM DTT). The cells were disrupted by an ultrasonic cell disruptor (“Bioruptor”). The undisrupted cells were removed by centrifugation (15,000 g, 60 min) to obtain a crude enzyme solution. The protein concentration in the crude enzyme solution was quantified by using a CBB solution (protein assay CBB solution, product of Nacalai Tesque) with bovine serum albumin as a standard sample.

By SDS-PAGE, the protein in the crude enzyme solution was separated. The specific procedure of the separation will be described next. The crude enzyme solution was mixed, after concentration adjustment, with a sample buffer (“Laemmli sample buffer”, product of BIORAD) at 1:1. After heating at 95° C. for 2 minutes, the resulting mixture was applied to “Ready Gel J 10%” (product of BIORAD) so that the applied protein amount was 10 μg, followed by electrophoresis for 40 minutes in a MiniProtean III Cell phoresis tank (product of BIORAD) at a constant voltage of 200V. As an electrophoresis buffer, tris/glycine/SDS (BIORAD) was used. For staining, Coomassie Brilliant Blue (“Bio Safe Coomassei”, BIORAD) was used. The results are shown below in FIG. 7 (marker: product of BIORAD, precision plus protein standards (#161-0363)).

Lanes 1 to 5 of FIG. 7 show ATCC13869ΔsucA(pVK9), ATCC13869ΔsucA(pVK9-xfp), ATCC13869ΔsucA(pVK9-tac_xfp), ATCC13869ΔsucA(pVK9-PS2_xfp), and ATCC13869ΔsucA(pVK9-PS2_xpkA)), respectively. Compared with the control band (pVK9), approximately 90 kD band which corresponds to that of the phosphoketolase gene product becomes denser in the strain which was modified to enhance the activity of phosphoketolase. This clearly suggests that a sufficient amount of expression of the phosphoketolase gene occurs in the coryneform bacterium.

In accordance with Meile's method (L. Meile, L. M. Rohr, T. A. Geissmann, M. Herensperger and M. Teuber, J. Bacteriol. 183, 2929-2936(2001)), the enzyme activity of phosphoketolase gene was measured. After reaction with 0.075 ml of a crude enzyme solution (33.3 mM KPO₄ (pH 6.5), 1.9 mM L-cystein hydrochloride, 23 mM sodium fluoride, 8 mM sodium iodoacetate, and 27 mM D-fructose 6-phosphate) at 37° C. for 30 minutes, 0.075 ml of hydroxylamine hydrochloride (2M, pH 6.5) was added. The mixture was allowed to react for 10 minutes at room temperature and then was stained with 0.05 ml of 15% (wt/vol) of trichloroacetic acid, 0.05 ml of 4M HCl and 0.05 ml of FeCl₃.6H₂O (in 0.1 m HCl, 5% wt/vol). After removal of crystals by centrifugal separation, the OD505 was measured using an enzyme activity reader (“Spectra max 190”, product of Molecular Devices).

The results are shown below in Table 3. No enzyme activity was detected in control (pVK9), while enzyme activity was detected in the strains in which phosphoketolase activity had been enhanced. TABLE 3 pVK9 xfp PS2_xfp PS2_xpkA ΔABS/mg/hr Trace 1.00 2.68 2.38

Example 7 Evaluation of L-Glutamic Acid-Producing Ability of the Bacterium Having Enhanced Phosphoketolase Activity

(7-1) Evaluation Under a Condition Containing a Sufficient Amount of Biotin

By using the strain C. glutamicum ATCC13869 ΔsucA, the effect of the introduction of a phosphoketolase gene on the fermentation yield of L-glutamic acid was evaluated. The cells of the ATCC13869ΔsucA(pVK9), ATCC13869ΔsucA (pVK9-xfp), ATCC13869ΔsucA(pVK9-PS2_xfp) and ATCC13869ΔsucA(pVK9-PS2_xpkA) strains obtained by culturing on a CM-Dex plate medium were inoculated into the Flask containing 30 g of glucose, 1 g of KH₂PO₄, 0.4 g of MgSO₄, 15 g of (NH₄)₂SO₄, 0.01 g of FeSO₄.7H₂O, 0.01 g of MnSO₄.7H₂O, 13.7 ml of a soybean hydrolysate solution, 200 mg of thiamine hydrochloride, 300 mg of biotin, and 50 g of CaCO₃ in 1 L of pure water (adjusted to pH 8.0 with KOH) and cultured at 31.5° C. with shaking until the suger was completely consumed. After completion of the culture, the amount of accumulated L-glutamic acid and OD in the culture broth were measured. The results are shown in FIG. 8. The results show that all of the strains in which xfp gene is amplified show high L-glutamic acid yield compared to the control.

(7-2) Evaluation when Penicillin is Added

In order to evaluate under the equalized final OD, the proliferation of cells was stopped by adding penicillin G. The yield of L-glutamic acid was compared by adding penicillin G to the medium as described in the above (1) so that its final concentration was 4 U/ml at plural growth points when the OD became within a range from 10 to 14 after the culture was started. The results are shown in FIG. 9. These results show that even if the yield of L-glutamic acid was compared among them at a point when their final cell counts were equal, the L-glutamic acid yield was higher in the xfp gene- or xpk gene-amplified strains compared with the control strain. The above-described results show that the introduction of phosphoketolase is effective for improving L-glutamic acid fermentation yield.

Example 8 Effect of Expression of Xfp Gene of B. animalis on L-Glutamic Acid Accumulation by Pantoea ananatis Strain

pVK9-PS2-xfp plasmid expressing xfp gene of Bifidobacterium animalis and control pVK9 shuttle vector were introduced to NP106/RSFCPG L-glutamic acid producing strain by electroporation using Bio-Rad MicroPulser. NP106/RSFCPG strain was obtained by curing pSTVCB plasmid from AJ13601 strain (FERM BP-7207; European patent publication 1078989) by plating the AJ13601 strain on the medium which does not contain chloramphenicol as a selection marker.

To transform NP106/RSFCPG strain with pVK9-PS2-xfp plasmid or pVK9 vector, cells obtained by overnight culture on LBG-M9 medium (trypton (10 g/L), yeast extract (5 g/L), NaCl (5 g/L), glucose (5 g/L), 0.5×M9 salt solution) with addition of tetracycline (10 μg/ml) was diluted 1:100 by fresh LBG-M9 medium and incubated at 34° C. with aeration up to OD₅₉₅=0.6. Cells from 10 ml of the culture were harvested by centrifugation, washed three times with ice-cold de-ionized water and with ice-cold 10% glycerol and resuspended in 10% glycerol. 10 ng of plasmid DNA were added to cell suspension and electric pulse (E=20 kV/cm, t=5 msec) was applied. 1 ml of LBG-M9 medium was added immediately after electroporation. Cells were incubated at 34° C. under aeration for 2 hours and plated on solid LBG-M9 medium containing 40 μg/ml of kanamycin. Plates were incubated at 34° C. for 48 hours. Grown clones were inoculated in 2 ml of LBG-MES medium (trypton (10 g/L), yeast extract (5 g/L), NaCl (5 g/L), glucose (5 g/L), MES 0.1M, pH7.0) containing 40 μg/ml kanamycin and 10 μg/ml tetracycline and incubated at 34° C. under aeration overnight. 80 μl of the overnight-cultured medium containing the bacterial cells were inoculated in 2 ml of fermentation medium (glucose (40 g/L), MgSO₄.7H₂O (0.5 g/L), (NH₄)₂SO₄ (16 g/L), KH₂PO₄ (0.3 g/L), KCl (1.0 g/L), MES (10 g/L), betaine (1.0 g/L), FeSO₄.7H₂O (10 mg/L), MnSO₄.5H₂O (10 mg/L), lysine, methionine, and DAP 100 mg/L of each, trypton (1 g/L), yeast extract (1 g/L), calcium pantothenate (10 mg/L), CaCO₃ (30 g/L)) in test-tubes and incubated for 26 hours with aeration. The amount of accumulated L-glutamic acid was determined by the TLC method. Average data of 4 independent experiments are represented in Table 4. TABLE 4 Strain OD₅₄₀ Glu, g/L Yield, % NP106 RSFCPG/pVK9 21.8 9.8 24.4 NP106 RSFCPG/pVK9-PS2-xfp 20.7 12.4 30.9

As could be seen from Table 4, the strain carrying pVK9-PS2-xfp plasmid showed L-glutamic acid accumulation approximately 2.6 g/L higher than the control strain transformed with vector pVK9, which corresponds to at least a 6% increase in yield.

Example 8 Improvement of Fermentation Yield of L-Glutamic Acid by a Combination of the Enhancement of Phosphoketolase Activity and the Reduction of 6-Phosphofructokinase (PFK) Activity

8-1. Preparation of a sucA and PFK Double-Disrupted Strain

(1) Construction of a Plasmid for Disruption of pfk Gene

Gene fragments lacking the ORF of pfk derived from the strain ATCC13869 were obtained by the overlap PCR method using synthetic DNA as primers designed based on the nucleotide sequence of the corresponding gene of Corynebacterium glutamicum ATCC13032 (GenBank Database Accession No. NC_(—)003450; SEQ ID NO: 61). Specifically, the amplification products of the N-terminal region of the pfk gene were obtained by a conventional PCR method using chromosomal DNA of the C. glutamicum strain ATCC13869 as a template, and synthetic DNAs of SEQ ID NO: 50 and NO: 51 as primers. On the other hand, in order to obtain amplification products of the C-terminal region of the pfk gene, a conventional PCR was performed using chromosomal DNA of the strain ATCC13869 as a template and synthetic DNAs of SEQ ID NO: 52 and NO: 53 as primers. The sequences of SEQ ID NO: 51 and NO: 53 are complimentary to each other.

Then, in order to obtain a pfk gene fragment in which its internal sequence is deleted, the aforementioned amplification products of the N-terminal and C-terminal region of the ptk were mixed in substantially equimolar amounts, and gene amplification products were obtained by a conventional PCR using this mixture as a template and synthetic DNAs of SEQ ID NO: 54 and NO: 55 as primers. The PCR product was purified in an ordinary way and then digested with BamH I, and was inserted at the BamH I site of the above-described pBS5T. This DNA was used to transform competent cells of Escherichia coli DH5α (product of Takara Bio Inc.), and the cells were plated and cultivated overnight on LB medium supplemented with 100 μM of IPTG, 40 μg/ml of X-Gal, and 25 μg/ml of kanamycin. Then, white colonies that emerged were picked and separated into single colonies to obtain transformants. Plasmids were isolated from the obtained transformants, and the plasmid having an insert of the target PCR product was selected and named pBS5T-Δpfk.

(2) Preparation of a Disrupted Strain

The aforementioned strain ATCC13869ΔsucA was first transformed with a high concentration of the plasmid pBS5T-Δpfk by the electric pulse method, and plated on CM-Dex medium (5 g/L of glucose, 10 g/L of polypeptone, 10 g/L of yeast extract, 1 g/L of KH₂PO₄, 0.4 g/L of MgSO₄.7H₂O, 0.01 g/L of FeSO₄.7H₂O, 0.01 g/L of MnSO₄.7H₂O, 3 g/L of urea, 1.2 g/L of soybean hydrolysate, and 10 μg/L of biotin, adjusted to pH7.5 by NaOH) supplemented with 25 μg/ml of kanamycin, and cultivated for about 60 hours at 25° C. The resultant transformants were cultivated with shaking overnight at 34° C., and then suitably diluted, and plated and cultivated for 30 hours on CM-Dex medium supplemented with 25 μg/ml of kanamycin. The strain that can grow on this medium is a strain in which the kanamycin-resistance gene and the SacB gene derived from said plasmid are integrated into its chromosome, as a result of homologous recombination between the pfk gene fragment on said plasmid and the same gene located on the chromosome of the strain ATCC13869.

Then the first recombinants were cultivated overnight in kanamycin-free liquid CM-Dex medium at 31.5° C., and after an appropriate dilution, they were plated on kanamycin-free and 10% sucrose-containing Dex-S10 medium (10 g/L of glucose, 10 g/L of polypeptone, 10 g/L of yeast extract, 1 g/L of KH₂PO₄, 0.4 g/L of MgSO₄.7H₂O, 0.01 g/L of FeSO₄.7H₂O, 0.01 g/L of MnSO₄.7H₂O, 3 g/L of urea, 1.2 g/L of soybean hydrolysate, 10 μg/L of biotin, and 2 g/L of sodium acetate, adjusted to pH7.5 by KOH), and cultivated for about 30 hours at 34° C. As a result, a strain was obtained that seems to have lost the SacB gene and become insensitive to sucrose because of the second homologous recombination.

The obtained strains include those having the disrupted type of pfk gene derived from pBS5T-Δpfk, and those having the wild-type pfk gene. To confirm whether the pfk gene was disrupted type or wild-type, direct PCR analysis was performed using the bacterial cells cultivated on a Dex-10 agar medium. The strain having only the disrupted-type of pfk was selected and named ATCC13869 ΔsucA Δpfk.

8-2 Acquisition of Mutant pfk Genes Encoding PFK Having Reduced Activity

(1) Mutant PFK Having Reduced Activity

As PFK with reduced activity, PFK*1(SEQ ID NO: 57) and PFK*2(SEQ ID NO: 59) were obtained. PFK*1 is generated by substituting lysine for glutamic acid at position 171 of the wild-type PFK (SEQ ID NO: 61). PFK*2 is generated by substituting arginine for glutamic acid at position 171 of the wild-type PFK (SEQ ID NO: 61). These sequences can be obtained by crossover PCR using the wild-type pfk gene as a template and synthetic DNAs for introducing such a mutation as primers.

(2) Construction of Expression Vector for the Mutant pfk Gene and the Phosphoketolase Gene

In order to construct a PFK expression vector, a conventional PCR was performed using chromosomal DNA of the C. glutamicum strain ATCC13869 as a template, and synthetic DNAs of SEQ ID NO: 54 and NO: 55 as primers. The PCR product was purified in an ordinary way and then digested with Xba I and Kpn I, and the gene fragment containing the pfk gene was inserted at the Xba I-Kpn I site of the above-described pVK9. This DNA was used to transform competent cells of Escherichia coli DH5α (product of Takara Bio Inc.), and the cells were plated and cultivated overnight on LB medium supplemented with 100 μM of IPTG, 40 μg/ml of X-Gal, and 25 μg/ml of Km. Then, white colonies that emerged were picked and separated into single colonies to obtain transformants. Plasmids were isolated from the obtained transformants, and the plasmid having an insert of the target gene was selected and named pVK9-PFK.

Then, in order to construct a plasmid that expresses both the mutant PFK and the phosphoketolase at the same time, the above-mentioned pVK9-PS2_xfp was digested with Xba I, and then the gene fragment containing the PS2_xfp was purified and inserted into Xba I site of the pVK9-PFK. This DNA was used to transform competent cells of Escherichia coli DH5a (product of Takara Bio Inc.), and the cells were plated and cultivated overnight on LB medium supplemented with 100 μM of IPTG, 40 μg/ml of X-Gal, and 25 μg/ml of Km. Then, white colonies that emerged were picked and separated into single colonies to obtain transformants. Plasmids were isolated from the obtained transformants, and the plasmid having an insert of the PS2_xfp and pfk was selected and named pVK9-PS2_xfp_PFK.

In order to construct a plasmid that expresses both the mutant pfk gene and the phosphoketolase gene at the same time, pVK9-PS2_xfp_PFK*1 and pVK9-PS2_xfp_PFK* were generated according to the same procedure as employed in construction of pVK9-PS2_xfp_PFK.

(3) Analysis of the Activity

The aforementioned expression plasmids for PFK and phosphoketolase (pVK9-PS2_xfp_PFK, pVK9-PS2_xfp_PFK*1, and pVK9-PS2_xfp_PFK*2) were used to transform the above-mentioned strain ATCC13869 ΔsucA Δpfk to examine the enzymatic activity of PFK. The transformation was performed by the electric pulse method, and the transformants were obtained by plating the bacterial cells on CM-Dex medium (5 g/L of glucose, 10 g/L of polypeptone, 10 g/L of yeast extract, 1 g/L of KH₂PO₄, 0.4 g/L of MgSO₄.7H₂O, 0.01 g/L of FeSO₄.7H₂O, 0.01 g/L of MnSO₄.7H₂O, 3 g/L of urea, 1.2 g/L of soybean hydrolysate, and 10 μg/L of biotin, adjusted to pH7.5 by NaOH) supplemented with 25 μg/ml of kanamycin, and cultivating them for about 30 hours at 31.5° C.

Crude enzyme solution was obtained in the following way. Bacterial cells in a logarithmic phase were collected, washed by 100 mM Kpi buffer (pH8.2), and then dissolved in the same buffer, followed by ultrasonic disruption. The soluble fraction was separated by ultracentrifugation (60000 rpm, 30 min) to obtain crude enzyme solution. The procedure for quantifying protein concentration in the crude enzyme solution is shown below. The crude enzyme solution and the BSA solution of known concentration for generating a standard curve were each reacted with CBB solution (protein assay CBB solution, Nacalai Tesque) to develop color, and followed by measuring OD595 nm using spectra max 190 (Molecular Divices).

Then, PFK activity was measured according to the conventional method (Denise Kotlars and Henri Buc, Methods in Enzymology (1982) 90: 60-70). The specific procedure is shown below. Enzymatic activity was determined by adding crude enzyme solution to 100 mM Tris-HCl (pH8.2), 10 mM MgCl₂, 1 mM ATP, 1 U/ml glycerol-3-phosphate dehydrogenase, 1 U/ml triosephosphate isomerase, 1 U/ml aldolase, 0.2 mM NADH, and 10 mM fructose-6-phosphate, followed by measuring time-course changes in OD340 nm using spectra max 190 (Molecular Divices). Relative activity of the reduced PFK when the activity of wild-type PFK is set at 1, is shown in the following Table 5. TABLE 5 pVK9-PS2_xfp_PFK pVK9-PS2_xfp_PFK*1 pVK9-PS2_xfp_PFK*2 Relative PFK activity 1 0.33 0.41

8-3 Increase in Yield of L-Glutamic Acid by Reducing PFK Activity

(1) Evaluation Under a Condition Containing a Sufficient Amount of Biotin

By using the strain C.glutamicum ATCC13869 ΔsucA Δpfk, the effect of reduction in PFK activity on the fermentation yield of L-glutamic acid was evaluated. Cells of each strain obtained by cultivating the strain on a CM-Dex plate was inoculated into a medium containing 30 g of glucose, 1 g of KH₂PO₄, 0.4 g of MgSO₄, 15 g of (NH₄)₂SO₄, 0.01 g of FeSO₄.7H₂O, 0.01 g of MnSO₄.7H₂O, 13.7 ml of a soybean hydrolysate solution, 200 μg of thiamine hydrochloride, 300 μg of biotin, and 50 g of CaCO₃ in 1 L of pure water (adjusted to pH 8.0 with KOH), and cultivated at 31.5° C. After completion of the culture, the amount of accumulated L-glutamic acid and bacterial cell concentration (OD) in the culture broth was measured. The result is shown in FIG. 10. The result shows that the strain expressing the mutant pfk gene exhibited higher L-glutamic acid yield compared to the strain expressing the wild-type pfk gene.

(2) Evaluation when Penicillin is Added

The yield of L-glutamic acid was compared by adding penicillin G to a final concentration of 4 U/ml to the above-described medium in order to stop cell growth to obtain an equalized final bacterial amount, at plural growth points when the OD became within a range from 10 to 14 after start of the culture. The result is shown in FIG. 11. The above-described results show that the reduction in PFK activity in phosphoketolase-introduced strains is effective for improving fermentation yield of L-glutamic acid.

Example 9 Evaluation of L-Glutamine-Production using a Bacterium in which Phosphoketolase Activity is Enhanced

By using the strain B. flavum AJ11576 (JP56-16479A), the effect of the introduction of a phosphoketolase gene on the improvement of fermentation yield of L-glutamine was evaluated.

The AJ11576 strain is resistant to compounds which have vitamin P activity. This strain was deposited at the National Institute of Bioscience and Human-Technology, Agency of. Industrial Science and Technology, Ministry of International Trade and Industry (currently, National Institute of Advanced Industrial Science and Technology, International Patent Organism Depositary, Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan) and received an accession number of FERM BP-10381.

The pVK9 (plasmid for control) and pVK9-PS2_xfp (plasmid for amplifying xfp gene of Bifidobacterium animalis, in which the native promoter is replaced with a PS2 promoter) were each introduced into the above-described strain B. flavum AJ11576 to obtain phosphoketolase-expressing strains. Specifically, the strain was transformed with the each of the plasmids by the electric pulse method, plated on a CM2G medium (5 g/L of glucose, 10 g/L of polypeptone, 10 g/L of yeast extract, 5 g/L NaCl, adjusted to pH 7.0 by KOH) containing 25 mg/ml of kanamycin and cultured at 31.5° C. for about 30 hours. The emerged colonies were isolated as transformants and designated AJ11576 (pVK9) and AJ11576 (pVK9-PS2_xfp), respectively.

The cells of the AJ11576 (pVK9) and AJ11576 (pVK9-PS2_xfp) strains obtained by culturing on a CM2G plate medium were inoculated into the Flask culture medium containing 100 g of glucose, 2.5 g of KH₂PO₄, 0.4 g of MgSO₄, 60 g of (NH₄)₂SO₄, 0.01 g of FeSO₄.7H₂O, 5.7 ml of a soybean hydrolysate solution, 2 mg of thiamine hydrochloride, 4 μg of biotin, 0.02 ml GD-113 and 50 g of CaCO₃ in 1 L of pure water (adjusted to pH 6.8 with NaOH) and cultured at 31.5° C. with shaking until the sugar was completely consumed. After completion of the culture, the amount of accumulated L-glutamine and OD in the culture broth were measured. The results are shown in FIG. 12. It has been revealed that all of the xfp-gene amplified strains exhibited high L-glutamine production compared to the control strain.

While the invention has been described in detail with reference to preferred embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. All the references cited herein, including the priority documents, RU 2004124226 and U.S. 60/644,562, are incorporated as a part of this application by reference. 

1. A bacterium having an ability to produce a useful metabolite, wherein the bacterium is modified to have an increased activity of D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase.
 2. The bacterium according to claim 1, wherein the useful metabolite is derived from acetyl-coenzyme A.
 3. The bacterium according to claim 1, wherein the useful metabolite is selected from the group consisting of L-glutamic acid, L-glutamine, L-proline, L-arginine, L-leucine, L-cysteine, succinate, and polyhydroxybutyrate.
 4. The bacterium according to claim 1, wherein the bacterium inherently does not have an activity of D-xylulose-5-phosphate phosphoketolase and fructose-6-phosphate phosphoketolase, and wherein said bacterium is transformed with a DNA fragment coding for D-xylulose-5-phosphate phosphoketolase and/or fructose-6-phosphate phosphoketolase.
 5. The bacterium according to claim 1, wherein the bacterium is further modified to have a reduced activity of 6-phosphofructokinase.
 6. The bacterium according to claim 1, wherein the bacterium is selected from the group consisting of Enterobacteriaceae, coryneform bacterium, and Bacillus bacterium.
 7. The bacterium according to claim 1, wherein the bacterium belongs to the genus Escherichia or Pantoea.
 8. A method for producing a useful metabolite comprising cultivating the bacterium according to claim 1 in a culture medium, and collecting said useful metabolite from the culture medium and/or the bacterium.
 9. A method for producing a useful metabolite derived from acetyl-coenzyme A comprising cultivating the bacterium according to claim 2 in a culture medium, and collecting said useful metabolite from the culture medium and/or the bacterium.
 10. A method for producing a useful metabolite selected from the group consisting of of L-glutamic acid, L-glutamine, L-proline, L-arginine, L-leucine, L-cysteine, succinate, and polyhydroxybutyrate comprising cultivating the bacterium according to claim 3 in a culture medium, and collecting said useful metabolite from the culture medium and/or the bacterium.
 11. A method for producing a useful metabolite comprising cultivating the bacterium according to claim 4 in a culture medium, and collecting said useful metabolite from the culture medium and/or the bacterium.
 12. A method for producing a useful metabolite comprising cultivating the bacterium according to claim 5 in a culture medium, and collecting said useful metabolite from the culture medium and/or the bacterium.
 13. A method for producing a useful metabolite comprising cultivating the bacterium according to claim 6 in a culture medium, and collecting said useful metabolite from the culture medium and/or the bacterium.
 14. A method for producing a useful metabolite comprising cultivating the bacterium according to claim 7 in a culture medium, and collecting said useful metabolite from the culture medium and/or the bacterium. 